But what if I just told somebody how to replace, say, a fixed wall light? Would that be classed as "encouraging an unlicensed person" to carry out wiring?

The British Expats forum has been providing some quite interesting electrical threads in the Aussie/NZ section lately, and it's funny you should mention just changing a plug. Have a look at the lengthy argument which developed over just such a query:
http://britishexpats.com/forum/showthread.php?t=261775

(Yes, PBC_1966 there at the end is me -- I couldn't resist adding my 2 cents! [Linked Image])