Flickchick,
"i'm in melbourne suburbs and i have some young long neck turtles. just wondering at what age/size should i let them start hibernating."
Turtles grow at different rates, depending on their diet and how much they have been fed. Basically, for longnecks (Chelodina longicollis), once their carapace is about 10cm, they are big enough to survive the cold of winter outdoors.
"how do i do so, i know ur meant to not feed them for a few days/weeks before hand, but i'd have no idea when to stop feeding them."
Transfer them to a large outdoor pond, once the water temperature has risen to about the same temperature as your indoor tank. Gradually cool your indoor tank if you need to bring it down to match the pond temperature. Normally, the indoor/outdoor water temperature will be the same sometime in Spring. It depends on many factors, including how warm it is where you live, how much sun your pond gets, how deep it is and how warm your tank is.
Once they are in the outdoor pond, leave them they. The pond will gradually heat over Summer and then gradually cool as we head into Winter. The turtles will gradually and naturally cool down with the water temperature.
Once the water temperature drops to about 16ºC (probably around Easter time), stop offering them food. By the time the water drops to about 14ºC they will have lost interest in feeding anyway, and you'll be seeing them less frequently.
do they normally hibernat in water or soil.
Water. Especially if you have a big deep pond. If you have a little, shallow pond, they'll probably hibernate on land. But, I would never recommend keeping turtles in a little or shallow pond.
Regards,
Michael.