Has anyone else experienced a 'zap' when you touch the metal part on an iPhone 4 whilst it is on charge? If so, how do you fix it? or is it just another fault?

there is an "earthing" failure in the charger or the phone. Make sure that the wall outlet has an earth pin and that the charger plug has an earth pin. For example: in Switzerland the charger plug has 3 "pins", the middle one being the earth pin. Many just cut this middle pin off to use in another european country wall outlet. This can be dangerous.

difficult. make sure that you are not standing on an "isolating" floor like whool or synthetic or wearing rubber shoes and the like. It is really serious, do you not have earthed wall sockets at all? And is this the original charger?

I'm not familiar with the layout of AC outlets in Australia, but if it has just 2 pins, can the power adapter be turned over to be plugged in upside down. If so, give that a try.

Stedman

I have checked: there are two "systems" in your country:

a. the earthed wall socket has one pin (earth) and two holes. Your plug should have two pins and one hole (earth).

b. the socket has two holes and on the side two metal strips, while the plug has two pins and two metal strips on the side.

And there are combinations (like the most european countries). Make sure that you use an earthed wall socket.I supppose that it is not the charger plug when it is the original. In older houses the socket can be the "earthed" type while in there there is no earth wire, have an electrician check that.

If the earthing of plug and socket is not the cause, you should buy a new charger.

You should absolutely solve this.

When it is really two pins, turning around will not solve an earthing failure,

Earth is not the same as "live" and "phase" of the wires of the electricity.

OK when you have checked the "earthing" thing, I suppose it is the charger being faulty, not your phone. This is guarantee.