Avios should be good for domestic Australian flights on QF or QF flights between Australian and New Zealand, especially if you don't try to go near Perth.

I've been discouraged from trying to use trains in Australia. The service isn't frequent, distances are long, trains aren't very good and (I've been told by natives) tend to have accidents and major delays, etc. Exceptions would be a couple special infrequnet luxury tourist trains, of which the Orient Express along the east coast has apparently permanently suspended service.

Cairns is the easiest place to get into for the Great Barrier Reef. Green Island (coral) is only an hour away by hydrofoil. Cairns itself impressed me as somewhat of a backpacker destination but but there's a charming nearly coastal village or you can go up north to see nature.

When I was there, I arrived on a QF codeshare nonstop operated by JAL from NRT, so Cairns does have international long haul nonstop service. The old CO flew there from Guam IIRC, but I suspect UA has discontinued these flights. If you take domestic flights between secondary (or worse) airports, watch out for the tiny (QF commuter affiliate) aircraft with very restricted baggage limits, sometimes permitting only a small number of kilograms with no possibility of paying for any excess.