The problem you describe sounds like migration of the plasticiser in the pvc insulation. This occurs as a result of thermal cycling of the conductor due to changes in load and conductor temperature drawing the thermodynamically unstable insulation plasticiser to the copper conductor. The sticky blue/green liquid is not corrosive and can be cleaned with methylated spirits. There is no cure for plasticiser migration, other than replacement of the cable.
This migration only affects older PVC cable (pre mid 70's), as newer cables use refomulated pvc compounds.
I would strongly advise you to have a qualified electrician inspect & test your wiring to ensure that it is still in safe condition.
I have seen this problem several times, but usually the sticky liquid stays inside the mounting box.
David, Welcome to ECN, mate!. We also have this problem, over here in New Zealand, like yourself, I wondered what the H**L it was, when I first saw it, it had run down the wall, from the Earth aperture on the socket, it was like a cheap horror movie!.
I can't say this is a problem I recall seeing before.
I've come across some fittings on very damp outside walls where the green stuff was most definitely corrosive. In fact, in one case it had eaten through the earth wire and partially destroyed the insulation and wire on the phase, enough that a little movement left the metal box energized!
I was thinking Ectoplasm
I'm thinking of the cans of "Slime" that some of my school friends had in the late 1970s. Not sure what that stuff was made from.