It depends on your contract/award.

There's no automatic provision for unpaid leave in my position - it's just not something I'm entitled to - so it would have to be negotiated with my boss.

A colleague tried to get some not long ago, and was told no, essentially because then everyone would want it ... but maybe if he'd asked for less, or done it more privately it would have been granted ... ?

If you never ask you'll never know - ask and see what they say.