Went to try and book the tickets the other day, but the travel agent gave us quite a bit to think about. She said that for about another £200 we could do a round the world trip, which would be amazing but I'd need a lot more money for it, I'm more than happy just going to Australia right now.
She explained that we could book a return flight but leave from a different place that we flew into which is great because it means we won't have to make our way back down the country. Although because we're not certain of when we're coming back, it's not that much more just to buy two singles, so we can buy the return ticket nearer the time.
We're thinking of flying into Melbourne or Adelaide and then flying out from Cairns. It'll be such a great way to finish the trip, swimming in the Great Barrier Reef, just have to make sure we have enough money left for it all!