Hey Sam
I know how it feels to be small and carry great weights!
My pack last year was pretty light (probably just over 16kg) cause I went in a group with 9 people and as I had just injured my back the week before didn't have to carry a dinner. However at Nelson Lakes 2006 my pack was probably around 20kg. So you can get the weight down a bit.
The key thing on Mid-Winter is food. So try and get stuff that is as light as possible, ie crackers for lunch with salami and cheese, that can also go on dinner. Museli bars, and chocolate, and cereal or something for snacks and brekky. And for dinner try taking something like those continental pasta packets with dehy peas and corn for however many people end up in the group.
The lighter you can get things, the more fun the trip is!
Andrea