A common method is to fix one or two pieces of plaster board behind the hole(in the void above the ceiling ) , fixing it in place with plenty of plaster on all sides of the hole...then wait until it has gone off.
Apply PVA to blocked off hole...let it dry ...then plaster this blocked off hole in two stages to make a flush finish.
There are variations to this such as blocking off the hole from above with a piece of floorboards between the joists , then plastering the blocked off hole as before.
I`ve even used one of the cut out circles of plasterboard and screwed that to the piece of floorbard between the joist then skim plastered it.
As an electrician , I`ve done this many times.
David
March 2009
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