Looking somewhat like a distorted question mark from above (though the Tamarama Beach "overview" image (11689) is as close as you'll come to seeing that in this gallery), this radiate pine and screws work is by US sculptors Aaron Lee and Betty Jane Benson.

"The work is designed to respond to its environment as well as relate positively to the viewers".

It would also make a great play fort, I suspect. Might need a couple of watch towers and a closable gate, but aside from that...