How old are the handsets? If they are Mk1's, then they won't work with a new PSTN 2 base. If they are MK2's then they will work (with a PSTN 2 base), but may require a software update if they are early Mk2's (which is not as cheap as you may think). If you can find an Australian PSTN 1 base, then I 'think' you will be OK but I'm not 100% sure. I don't know about the wavelength issue.
It is a complete minefield. If you can afford to do it I would probably just sell what you have on eBay (B&O phones generally do quite well with a decent (technical) description and photos) and buy a totally new set in Australia. You get a new waranty, new battery and your 100% sure it will all work.
Ian