We booked through a US travel agent as they offer more flexibility. Due to sheer mass of the US consumer size then they are more accommodating than TA's in Australia and indeed cruise lines themselves. Not all cruise lines accept Aussies booking through a US Agent. Princess for instance.
US travel agents act as third parties but all financial transactions are made by the cruise line so perfectly safe. If the US TA goes bust, it doesn't affect your booking as the deposit has been taken by the cruise line so they still have your money.
The best thing - you can cancel up to 90 days before the cruise, for any reason, and receive your deposit back in full. No penalty whatsoever for cancellation. We cancelled a cruise last year and received more than we'd paid as the Aussie dollar had fallen. Try not being charged by an Aussie Agent! You lose the deposit in full which can be a few hundred dollars.
The downside - currency fluctuations. And you need to DYOR as sometimes the deals in Australia are better. Assuming you don't cancel.
I use Vacationstogo.com.