For starters, get a silk sleeping-bag liner. In the U.S., Design Salt makes an excellent one for about $60, and I know there are some Australian makers as well. This will serve as your sleeping "bag" in very hot climates.
Next, find a very lightweight sleeping bag rated to about zero-degrees Celsius (32-degrees Fahrenheit). I'm a little conflicted over whether to recommend synthetic or down fill. Down is lighter and more compressible, but also can absorb moisture in warm, wet climates. For this piece, though, I'll go with down. The New Zealand outdoor-gear maker, Macpac, makes a "summer" bag called the Dragonfly that would do nicely (NZ$449). It'll do well as you reach cooler temperatures, and as a bonus allows you to shift the down fill around inside the bag, so you can adjust the amount of insulation it provides.
The last piece you'll need is some sort of overbag to cover the Dragonfly (or its equivalent) when you're in very cold conditions. In North America, Canada's Mountain Equipment Co-op sells a nice one that uses synthetic insulation. Called the Penguin, it'll add five- to ten-degrees Celsius to the total sleeping-bag rating. Combine it with the silk bag liner, the sleeping bag, and a good sleeping pad, and you should be snug close to minus 17-degrees Celsius. Alternatively, buy an overbag that unzips into a blanket, and use it to put over the both of you. You'll sleep warmer side by side, anyway. Again, Macpac will help here with its Firefly (NZ$229), a lightweight overbag with a full zip so it turns into a duvet.
Hope that helps!
Recommended gear sites: Design Salt; Macpac; Mountain Equipment Co-op.