Complex sentence: Every series of "Big Brother Australia" included "Intruders": new housemates added to the house by the show's producers as an on-going housemate after the series has started.
Split into simpler sentences: Every series of "Big Brother Australia" usually includes "Intruders". Intruders are new housemates added to the house by the show's producers as an on-going housemate after the series has started.