I have seen people drive themselves half-mental trying to deal with small holes in plasterboard dry-lining, so here is the easy way:
Don't try to fix another piece of plasterboard to the back of the panel using plaster ! You will just end up with filthy fingers and about twenty little bits of board falling into the cavity.
Don't try Soudafoam expanding filler or any such stuff. This will usually fall off in hideous globs and descend into the cavity.
• Obtain some short lengths of 2" x 1/2" doorstop.
• Fix a screw into the centre of each piece to act as a handle.
• Negotiate two (or more) pieces of this wood into the hole, using the "handle" screw to pull tight against the panel.
• Fix through the panel into the doorstop using 3/4" stainless screws above and below the hole.
• Remove the "handle" screws.
• Repair with plaster to level.
Very easy, but you would be surprised how many people try other methods unsuccessfully !