Some folk like to lug around a lead sleeping bag. Might be useful in the event of a nuclear attack I guess. The bag I use is the Montbell Ultralight Superstretch Down Hugger #3 (-1C) which weighs about 600 grams. My wife has a Zpacks -7C sleeping bag which weighs even less than mine (400 grams)!
The thing I particularly like about the Montbell ‘Stretch’ range is their roominess. There are lots of other light ‘mummy’ down bags (around a pound (500 grams) but most are so narrow you have trouble even doing them up let alone having a comfy night’s sleep in them. The Montbell have enough stretch so you can sit up in them with your legs crossed! But they still ‘shrink’ to fit so there is no (wasted) air space your body has to heat up.
A down bag will weigh around half the weight of a synthetic bag of the same rating. Some folk worry about getting cold in them as they have the (undeserved) reputation of losing their insulative ability when wet. This is not true: Anyway, work at keeping your sleeping bag dry. You will be much more comfortable that way!
We use wool thermals & Montbell down clothes to increase the rating of our bags. A liner bag will increase the rating of your bag by 8C for only 248 grams weight penalty http://seatosummit.com/product/reactor-thermolite-mummy-liner/. This is less than half the weight penalty you would pay for buying a bag with an 8C lower rating – and you only need the one sleeping bag.
I always have an open shelter with a fire out the front. By the same token I usually sleep along the valleys rather than on the tops. This post might also be helpful: http://www.theultralighthiker.com/the-ultralight-deer-hunter/
I added a lot of down to my bag to increase its rating to -20C for my Everest trip http://www.theultralighthiker.com/adding-down-to-a-sleeping-bag/. It still only weighed approx 850 grams something like Montbell’s #1/2 – but mostly in Vic you are unlikely to encounter temps of less than -10C. I sleep out in the winter all the time and find that my #3 bag is fine with the additions I mentioned above.
I think the Montbell down trousers weigh 239 grams (I have never needed them), then I have a vest at 150 and a coat at 200 as well as the wool, but I can wear the clothes at other times as well. I just don’t get lugging the single use sleeping bag weight around all the time.
BTW: You can wash down bags in the washing machine and dry them in a tumble drier.
And Yes! Spot would like to sleep in my bag with me – but he has his own!
PS: If you can’t get what you want in Australia, you can get it through these guys: http://www.theultralighthiker.com/buying-gear-online/