Hi & welcome to the forum fellow Victorian
I'm thinking its gonna be bloody cold up Eildon way then.
I hope you have 0 Degree rated sleeping bags (at least)
Your mattress will be comfortable but inflatable mattresses
don't insulate the cold from the ground, so you will likely
need to put something under it like foam mats etc and even
a good woolen blanket over the top.
There is at least one thread on this subject here in the forum.
The trading post is good for some bargains usually and keep an
eye out at garage sales and local markets for people getting rid of gear.