Inclusion Criteria:  Patient has provided written informed consent Male or female aged ≥ 18 years or older at written informed consent Histologically proven FL grade 1-3A according to the current World Health Organisation classification (2016) including all morphological variants. The B-cell nature of the proliferation must be verified by the positivity with an anti-CD20 antibody No previous chemotherapy, or other investigational drug for this indication apart from focal RT Stage I disease not amenable to single-agent definitive-dose RT, stage II, III or IV (as per Ann Arbor criteria - see appendix 1), suitable for treatment with non-curative intent At least one site of radiographically measurable disease not previously irradiated (at least one bi-dimensionally measurable site of disease: nodal disease >1.5 cm or an extranodal lesion > 1.0 cm in longest perpendicular diameter)  Deemed to need treatment by treating Investigator. Reasons for treatment can include, but are not limited to:  Any nodal or extranodal tumour mass >7cm AND/OR multiple extranodal disease sites Involvement of at least 3 sites each with diameter >3cm Symptomatic splenic enlargement Organ involvement/compression Ascites or pleural effusion (LDH) elevated Presence of systemic symptoms Disease progression in preceding 3 months Evidence of marrow infiltration with marrow compromise. (e.g. haemoglobin, WBC or platelet count below lower limit of institutional normal range)  Adequate bone marrow function within 7 days prior to registration defined as:  ANC ≥1.0x109/L WBC ≥2.0x109/L Platelets ≥100x109/L (with no platelet transfusion in the preceding 14 days) Haemoglobin ≥90 g/L (with no red blood cell transfusion in the preceding 14 days) Unless these are attributed to bone marrow infiltration by lymphoma. In cases were one or more results are lower than those specified above due to bone marrow infiltration by FL, patient may be eligible following consultation with the CPI.  Adequate organ function within 7 days prior to registration, defined as  Total bilirubin ≤1.5 x upper ULN with the exception of patients with known Gilbert's syndrome may be included if their total bilirubin is ≤3.0 x ULN and direct bilirubin ≤1.5 x ULN) AST and(ALT) ≤3 x ULN Adequate renal function with serum creatinine ≤1.5 x ULN or CrCl ≥ 40mL/min (using Cockroft-Gault formula, see Appendix 2) For patients not receiving therapeutic anticoagulation: INR and aPTT ≤ 1.5 x ULN For patients receiving therapeutic anticoagulation: stable anticoagulant regimen ECOG performance status 0-2 (see Appendix 3) Life expectancy greater than 6 months  Patients of childbearing potential must adhere to the following:  Female patients of childbearing potential must have a negative serum pregnancy test within 7 days prior to registration Note: A woman is considered to be of childbearing potential if she is postmenarchal, has not reached a postmenopausal state (> 12 continuous months of amenorrhea with no identified cause other than menopause), and is not permanently infertile due to surgery (i.e., removal of ovaries, fallopian tubes, and/or uterus) or another cause as determined by the investigator (e.g., Müllerian agenesis). The definition of childbearing potential may be adapted for alignment with local guidelines or regulations. Female patients of childbearing potential must be willing to use 2 methods of birth control or be surgically sterile, or abstain from heterosexual activity for the course of the trial through to 18 months after the last dose of treatment. Women must refrain from donating eggs during this same period.  Note: The reliability of sexual abstinence should be evaluated in relation to the duration of the clinical trial and the preferred and usual lifestyle of the patient. Periodic abstinence (e.g., calendar, ovulation, symptothermal, or postovulation methods) and withdrawal are not adequate methods of contraception.  Women must not be breastfeeding during study treatment and for 18 months after the last dose of obinutuzumab/study treatment. Male patients must agree to use an adequate method of contraception for the course of the trial through to 18 months after the last dose of treatment. Men must refrain from donating sperm during this same period. Able to comply with the study protocol requirements and follow-up procedures.  Exclusion Criteria:  Patient has grade 3B FL, transformed FL or other indolent lymphoma Requirement for urgent treatment due to life-threatening complications of the disease, for example: Compressive symptoms due to disease (which may or may not be bulky), such as superior vena caval obstruction; significant organ involvement causing compromise of organ function (including but not limited to liver/ renal obstruction, actual or impending spinal cord compression, uncontrolled pleural/pericardial effusions), malignant, symptomatic hypercalcaemia Central nervous system, meningeal involvement, cord compression from lymphoma Prior therapy with anti-PD-1, anti-PD-L1, anti-PD-L2, anti-CD137, or anti-CTLA-4 antibody or any other antibody or drug specifically targeting T-cell co-stimulation or checkpoint pathways  Treatment with systemic immunosuppressive medication (including, but not limited to, corticosteroids, cyclophosphamide, azathioprine, methotrexate, thalidomide, and anti-TNF- α agents) within 2 weeks prior to initiation of study treatment, or anticipation of need for systemic immunosuppressive medication during study treatment, with the following exceptions:  Patients who received acute, low-dose systemic immunosuppressant medication or a one-time pulse dose of systemic immunosuppressant medication (e.g., 48 hours of corticosteroids for a contrast allergy) are eligible for the study after Medical Monitor confirmation has been obtained Patients who received mineralocorticoids (e.g., fludrocortisone), corticosteroids for chronic obstructive pulmonary disease (COPD) or asthma, or low-dose corticosteroids (≤ 10mg prednisolone for orthostatic hypotension or adrenal insufficiency are eligible for the study  Patients with active, known or suspected autoimmune disease, with the following exceptions:  Well controlled type I diabetes mellitus Coeliac disease Residual hypothyroidism due to autoimmune condition only requiring hormone replacement Eczema or vitiligo or psoriasis not requiring systemic treatment and rash covering <10% of body surface area Other conditions not expected to recur in the absence of an external trigger Past history of pneumonitis or lung disease including idiopathic pulmonary fibrosis (including pneumonitis), drug-induced pneumonitis, organising pneumonia (i.e., bronchiolitis obliterans, cryptogenic organising pneumonia) Significant cardiovascular disease (such as New York Heart Association Class II or greater cardiac disease, myocardial infarction, or cerebrovascular accident) within 3 months prior to registration, unstable arrhythmia, or unstable angina Prior organ transplantation or allogeneic bone marrow transplantation Severe active infection with 4 weeks prior to registration , including, but not limited to, hospitalisation for complications of infection History or current evidence of any condition, therapy, or laboratory abnormality that might confound the results of the trial, interfere with the patient's participation for the full duration of the trial, or is not in the best interest of the patient to participate, in the opinion of the treating Investigator Known psychiatric or substance abuse disorders that would interfere with cooperation with the requirements of the trial Is pregnant or breastfeeding, or expecting to conceive or father children within the projected duration of the trial, starting with the screening visit through 18months after the last dose of trial treatment, for women and men respectively. Women of childbearing potential must have a negative serum pregnancy test result within 7 days prior to registration. History of HIV (HIV 1/2 antibodies) Active Hepatitis B (Patients with a negative hepatitis B surface antigen (HBsAg) test and a positive total hepatitis B core antibody test (HBcAg) at screening are eligible for the study provided that the screening hepatitis B virus (HBV) DNA test is negative or undetectable). Patients with known hepatitis B on current antiviral therapy are excluded Active Hepatitis C (Patients are eligible with a negative hepatitis C virus (HCV) antibody test at screening, or positive HCV antibody test followed by a negative HCV RNA test at screening. The HCV RNA test will be performed only for patients who have a positive HCV antibody test) Administration of a live, attenuated vaccine within 4 weeks prior to initiation of study treatment or anticipation that such a live attenuated vaccine will be required during the study. Influenza vaccination should be given during influenza season only (example: approximately March to October in the Southern Hemisphere). Patients must not receive live, attenuated influenza vaccine (e.g., FluMist®) within 4 weeks prior registration or at any time during the study treatment or within 5 months after the last dose of protocol treatment. B-cell recovery should be documented prior to administration of live vaccines. Has a known history of active TB (Bacillus Tuberculosis) History of severe allergic anaphylactic reactions to chimeric or humanised antibodies or fusion proteins Known hypersensitivity to Chinese hamster ovary cell products or to any component of the atezolizumab formulations Major surgical procedure, other than for diagnosis, within 4 weeks prior registration History of malignancy within 5 years prior to screening, with the exception of malignancies with a negligible risk of metastasis or death (e.g., 5-year OS rate > 90%), such as adequately treated carcinoma in situ of the cervix, non melanoma skin carcinoma, localized prostate cancer, ductal carcinoma in situ, or Stage I uterine cancer -