I was there (Perth) in November 2019 (see my signature for a link to the thread (this is the start of my Perth adventure:
Two Months of Lifers Down Under). It was unseasonally hot and very humid. Between the worst of the devastating bush fires). November the breeding season is in full swing. So I'd say October / November was the best time to go. Many of the lakes in Perth dry up over the summer.
I had a great time though. We have an excellent article in The Opus written by a member who lives in Perth
Perth area - BirdForum Opus. I'm sure
@davidfree would give you lots of advice too if you contact him.