YHA membership

I think that membership of the UK branch will cover you for hostel use in Australia and NZ. The reverse certainly works.

Are you a YHA member already? If so, you don't need to buy another membership in Australia.

If you're not already a YHA member, get it in the UK before you go -- it will be good for use in the rest of the world.

To answer your question, the YHA head office is (or at least was) in Sydney. It's likely that you'll be spending time in Sydney, so should be easy to buy a membership there. Check the YHA Australia website for address/contact details.

when I was there you could buy at any hostel. Check how many nights you'll need to stay in YHAs before it is worthwhile. We found it very difficult to get rooms in them, and rapidly got fed up with the noise.

with the Aussie weather, I recommend camping at the well equipped sites!

yha charge most. but u get monies worth. but pretty important you book ahead online asap. be aware re. community fridge, when grocery shopping that many fellow hostelers are low on funds and might just be out of bread/jam nudge nudge say no more.