Transit visa needed for Australia when flying from New Zealand to South Africa?

In general you do not need a Visa when simply transiting through most airports in Australia. Transit is air-side, and whilst you do need to pass through security screening, you will not pass through either customs or immigration. This falls under what is called "Transit Without Visa".

However officially there is an 8 hour time limit between scheduled arrival and scheduled departure for it to be considered a Transit Without Visa. In your case the 14 hours between flights obviously takes you over this 8 hour limit, so officially you DO need to obtain a "Transit Visa" in advance of arriving in Australia.

Presuming you stay within the airport there is nobody that will actually check that you do have a visa, however it's likely that the airline that is flying you NZ to Australia will not allow you to fly if they notice the extended layover and the fact that you do not have an Australia visa of some form.

Transit visas are free and normally relatively easy to obtain. It will allow you to remain in Australia for up to 72 hours, either remaining inside the airport or passing through customs/immigration and entering the country.

A 14 hour layover also implies you might be spending the night in the airport. I'm fairly sure that Melbourne does allow this, but I'd suggest checking. Some other airports (eg, Sydney) do NOT allow people to stay in the terminal air-side overnight, and thus you would definitely need a visa in order to leave the terminal.