It's a kite; my very strong inclination is to say Black Kite (mapped as wintering in the Perth area), but that's with my usual proviso that I'm not familiar with Australian natives and don't have an Aussie field guide, so only going by online pic searches. Square-tailed I'd think can be excluded as that has much more obvious barring on the primaries, and Whistling I'd think can be excluded as that has a rounded (not triangular-cornered) tail and prominent very pale inner primaries. Black-breasted Buzzard has a much heavier bill, and also white bases to its primaries.
Minnesippi. I had a close look at the spotted harrier and it seems a less bulky than this bird and this one's legs weren't prominent in flight.
Resin. I'm planning on going back down there this morning and will see if I can get some clearer shots. I just couldn't narrow it down from my ID books, but you're right about the triangular cornered tail.
The new photos are a different bird (or birds!) - these are a Whistling Kite, compare the rounded tail with the triangular tail in pics 2 & 4 in the top post; also the difference in the pale inner primaries not shown in the first set, and the streaked, rather than spotted, breast.
Thanks, Resin and Old Ned. That shows the differences very clearly Resin. I did notice the difference in the tails, and wondered if I was seeing two different birds, but didn't pursue that thought any further. I didn't notice the wings - I just thought I'd captured more detail because I was closer.