GO WITH THE CLIMATE !!!
#1a It will make your vacation more comfortable and enjoyable (heat exposure)
#1b It will make a huge difference to your travel costs
#1c It will prevent a lot of mozzie bites (mosquitos, midges insects, Stechfliegen)
The worst bites are from midges. They are so small you won't even notice them 0.5 to 4 mm. Some of them will even get through your mesh. But during the colder months and regions there are not too many around. Read more at https://www.australiawidefirstaid.com.au/biting-midges/ (source of the above picture (clipping).
If you travel with a campervan you will not have aircon and or a heater. Pack for hot days and could nights (zero degrees and below) if you travel south or into the outback. If you exclusively stay in the sub tropical region (such as Northern QLD) then a t-shirt and board shorts will suffice.