Holidays Act 2003, s 68
Your employer can require you to provide proof that you were sick or injured if the sickness or injury lasts for three or more calendar days in a row, including weekends and any other days that aren’t usually working days for you. This means they can require proof if, for example, you’re off sick on a Friday and then also the following Monday.
As proof you can provide a certificate from a doctor or nurse, or from another type of registered health professional as appropriate, like a physiotherapist if it’s an injury.
If the sickness or injury lasts for less than three days, your employer can only require you to provide proof if they pay for the reasonable cost of getting the proof – like the cost of going to your GP. Your employer must also tell you as early as possible that they want you to provide the proof.