A piece of ply (or similar), wider than the hole, with dabs of no more nails on each end, slotted through the hole, pulled tight to the back of the plasterboard and left to dry should do it.
To pull it tight, knock a small nail into the center of the ply and pull the head of the nail towards you with your fingers - or maybe tie some string round the head of the nail and pull tight (you may be able to tie the other end off around a fixed point opposite the hole to keep it tight for a while).
If you can make sure that the nail head does not jut out in front of the wall you can just plaster over it, otherwise you can probably just pull it out after plastering and smooth over the remaining nail hole.
RM