Bushwalk Australia

Overland track May 28th till June 3rd

tossing up weather to get a new sleeping bag or not. My current one is about 10-15 years old(handed down from dad), its a mountain designs tapered bag with a hood and its 100% down fill. I can not find any tags on it saying what it's rated too or the model. It lives in my swag and I've never had a cold nights sleep in it. I'm planning on sleeping in thermals/beanie and down jacket and fleece pants if need be. I've also got a S2S thermalplus liner. I've put it in a compression bag and weighed it 1.8kg.

I am tossing up weather I should buy a different sleeping bag that weighs a lot less (hoping for around the kilo mark) and one that i'll actually know the rating off.
My sleeping mat it the exped winterlite synmat, R-value 4.9.

So far I've been impressed by MONT brand
http://www.mont.com.au/helium-450-lhz-1 (comfort rated -1 for women, -7 for men, 800+ loft, 800g)
http://www.mont.com.au/helium-600-lhz-1 (comfort rated -4 for women, -10 for men, 800+ loft, 950g)

Pros/cons/suggestions?

They are both reasonably lightweight bags by the look of it. I would choose the Helium 600 because I'm a very cold sleeper but you might be different, my 600g fill sleeping bag (not a Mont) is warm enough for winter in Tasmania. They are made of much lighter fabric than your current one probably is, so that might be a consideration depending how much abuse you subject your gear to. The plus side of that is they weigh half as much and will take up a lot less room in your pack. There are lots of similar sleeping bags to choose from these days, I think it's about deciding how much down fill you need for your intended use and how warm/cold you sleep, how much your prepared to spend and what's readily available locally (unless you order online from overseas, not great the moment in terms of the dollar).

Mont make very good gear, which is designed for Australian conditions. I have one of their older bags and it's very warm (although it doesn't get much use since I got the ApII).

One of those would be perfectly adequate given the rest of your gear. Like Hermione, of the two I lean to the 600 - it's better to have a warmer bag and remove clothing than to be too cold for comfort.

You might also want to have a look at the S2S bags. They use 900+ down and the bags are well made.

The revamped One Planet range is well worth a look. Good products at fair and competitive prices.