Each brokerage has their own rules, in addition to exchange...

Each brokerage has their own rules, in addition to exchange rules, and I'm not an expert on all ASX rules, but as a general rule, the big "gotcha" about borrowing shares is that, like all things borrowed, you pay interest on the borrowing, so it is not free, and the longer you hold the short position, the more expensive it becomes. In the event of dividends, the short holder also has to cover the dividend payment.

Here in the States, there is one other issue to contend with. I don't know if it works the same in Oz, but here, the actual owner of the shares can decide that they want to sell their shares, in which case the shorter must cover within either 24 or 48 hours. They can do that by borrowing more shares to return to the original owner of their short position shares or they can close the position, but either way there are transactional costs.

In theory, you can hold the short position open forever, but it is not free.

Hope that helps, and have a nice weekend.