Hi there! Haven't been on line much lately so missed your post earlier. I'm in the SW, inland from Bunbury but used to live in Perth.. I'm not an experienced beek but here are my thoughts.
Are you intending to make new hives from your current ones? If so, I'd make sure there were enough pollen producing plants flowering and there was a nectar flow so the hive was bringing in good supplies; otherwise you might find the loss of workers as well as insufficient supplies caused loss of your main hive. I'd only divide if the hive was a strong one too: I'd wait until I was sure she was laying well and I can see brood at different stages of development. That means an increase in daylight hours and warmer temperatures.
You'd be taking out brood and honey so I'd want plenty on board.
Down here I'd wait until October I think, but perhaps wait until September is well advanced and temperatures have become steadily higher. And rain is less frequent.
Will you allow the colony to re-queen itself or buy and introduce a new queen to the nucleus?
Do you have a number of hives? Can you take a frame from each hive to build the new one?
I hope these thoughts have helped. I know I haven't answered you exactly, but my guess would be, wait until late September in Perth.
Cheers - Hollie