Soft Ceiling - Can't Install Ceiling Light

Hi guys,
I've just recently bought a house and has been doing some painting and changing lights. I've managed to installed all the lights except one in the lounge room.

Here is my problem:
I need to drill 3 holes in the ceiling in order to mount the new light fittings. The ceiling is plaster-based with threads in it ( I don't even what it is), unlike the current gyprock. If i pull those threads then I'll see more cracks in my ceiling. I've been drilling a few holes but everytime I put a screw in a plasterboard plug and screwing the fitting down, the plug would comes off as the area around the hole (where the electrical cable comes out) are too soft. I've try pushing it with my fingers and noticed it is soft only around 15cm around the hole. I really want to put the light in that area to covered up the existing hole and cracks from previous light fittings. Any help would be appreciated.:bawl::bawl:


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