Hi Ed
Girraween and Bald Rock are in the Granite Belt/New England regions of south Queensland and northern NSW and in winter they can be quite cold. The region around Stanthorpe near Girraween National Park regularly posts the lowest temperatures in Queensland (it snows) so it would most likely be a good idea to check the Bureau of Meteorology to check what conditions are like before you go. However it is best to expect and prepare for the cold with temps below zero over night. A good sleeping bag, and perhaps blankets or a doona if you are car camping, lots of layers as the days can still be quite warm.
Girraween has a number of large camp sites, however its best to call the ranger before hand for information. The website for Girraween is
http://www.derm.qld.gov.au/parks/girraween/index.html. The camp sites at Girraween have toilet and shower facilities though I personally have not stayed the night there to comment on their condition, though driving by the facilities look modern. I think one of the camp areas in Girraween is actually called Bald Rock Creek camp ground which can cause mistakes between groups of friends planning to meet up at either Bald Rock National Park or Girraween’s camp ground! (yep, personal experience!). I think there might be limited bush camping but you need to ask National Parks.
Bald Rock National Park is actually the same patch of bush but on the NSW side of the border. It has a smaller camp ground of 20 to 30 sites. Facilities are in the form of two pit toilets and a small kitchen set up and camp sites have fire places with timber supplied by National Parks. I think it is a nice quite place to camp in the region for a few nights.
The walks in both Bald Rock and Girraween are well marked and maintained. The region is characterised by granite out crops, of which Bald Rock is the largest. Most tracks originate from the camp grounds. The only issue between staying at one park or the other is that there is no marked track linking both parks and it is ~1 hour to drive from one to the other.
Hope that helps
Cheers
Andrew