In Mandurah? Not sure, it can get pretty chilly. I think down around the low 10s in celsius. At night it can drop a bit below zero, but not much. renaeden would know more having lived here longer.
Back in Geelong (my hometown), it can pretty damn cold, average of single digits (again celsius) during the day and below zero at night. During winter. In Geelong, during summer the highest recorded temp I believe was 42 celsius, though the average is usually high 20s.