As Ving has said a few of us at M.A.S. use the freezer suits that you see people use in the freezer units at super markets.
Hepworths are the suppliers here in Sydney ( Ingleburn ).
You can buy the suit as a one piece or as seperate pants and coat, they both come with a lined hood.
They also sell lined boots ( very comfy & warm ) called Huskies.
Suit and huskies were about $180. huskies about $50.
I used my gear at our very dark sky site last winter at Berrima and again at the Magellan Observatory, Goulburn. The temps were hovering at about -5c and I was as warm as toast

John