If a cafe (or similar) I would ask if it okay to leave the book. I have never had a refusal, although one person once put a limit of two books. Often the staff claim the book saying they want to read it. (I wish though they had been as eager to log it, as despite saying they would log it, they never have. But at least the book went to a new reader.)
I don't leave books too close to rubbish bins, as it might be tempting for someone to throw it out. I mean, I once had a book picked up and posted by someone. But at least that one was logged by the finder in the mail sorting, one or two hundred kms away :) They were as surprised to find the book in the mail, as I was to read it had been posted.
I can't imagine the beach having cleaners/security, but then, I don't know the beaches of Perth. The amphitheatre sounds a good place. Also benches, stone walls, boulders and the like are good.
In places like that I usually pick the place because it's photogenic, as I like to take release photographs. In front of the ocean, or a colourful cafe/shop, etc.
Good luck and I hope the books are journalled.
There are release challenges you can join if they interest you. https://www.bookcrossing.com/---/23
My personal favourites are: Theme releases 2018 (I would have to include that, as it's mine :), & 52 towns in 52 weeks challenge 2018. If you travel the last one is particularly good.