Hi,
Not sure if the Acorn Archimedes ever made it to Australia, but if so, you can often find cheap Acorn-branded monitors that are re-badged Microvitec 1438s, and compatible with all OCS/ECS/AGA modes including those at 15KHz. The monitors are a bit basic, but will go up to 800x600 @ 60Hz.
Check eBay and any local education establishments!
Or, if you can persuade an NZ seller to ship to Australia, check
TradeMe (and in this case, the monitor comes with a free computer!)
- Ali