All you snowgoers, 
These three campsites are a great alternative to paying hundreds of dollars for accomodation in Jindabyne or the resorts. No site fees apart from  the N.P entry fee. But reccomended for tough, experienced campers as overnight winter temps can drop well below zero.
Coming from Jindabyne along the Alpine Way, Thredbo Diggins is reached soon after the N.P visitor entry station. This is a nice campsite down on the Thredbo River. There is plenty of room for many groups. The site has toilettes, open fire pits and bbq hot plates. There is a 6km historic walking track loop.
If you want a quieter site, then continue further down to Ngarigo. When i went in mid July, Ngarigo was empty, whilst there were 6 groups over at Thredbo Diggings. This is also a nice site down by the Thredbo River with similar facilities. It also is large enough for many groups. Its best feature is that it is just a five minute drive from Thredbo!
If you want even more isolation then continue past Thredbo, for about fifteen minutes or snow (longer in snowy, icy conditions), and the road descends steeply down to Leatherbarell Creek, where there is a smaller campsite. This has toilettes but only one firepit (there may have been a few more). If there has been lots of snow, then Leatherbarell Creek is the only site that will most likely be below the snowline - and its certainly a lot warmer since it is sheltered down by the river.
NOTE: If at Leatherbarell Creek and there is the possibilty of overnight snow on the road back up to Thredbo, plan to leave early since the N.P Patrol car comes at around 8:00am and decides if the road needs to be closed for the snow plough to come through. This could result in delays of up to an hour. 
If visiting Perisher Blue, then Island Bend is a similar alternative, just down the road from Guthega. Someone has already added this campsite.