When my friends go abroad I just text them as normal and it gets routed through provided they are in a place that has a network link - I am with O2 but it works with friends on other networks too.
My daughter, who visited Oz in 2011 is on o2 as am I. We think it cost about 50p a text and didn't get included in your free texts tariff. She thinks that you can buy a special deal for about £10, to enable you to have free texts while abroad.
No codes are necessary and you just text as normal.
If it`s a UK phone (that is in Australia) you just text as normal. You only do country codes if you are texting a foreign phone. It won`t cost you any more to text but it will cost the person more to text you and to receive your texts (for example I`m on T Mobile and with my UK phone when abroad it costs me 40p to send and receive texts)
You really need to phone your local o2 and explain as it also depends on the type of plan you're on. You may have to ask to go on international roaming or auto roaming.
With all the competing companies, check out if it is cheaper for your friend to buy a sim when in Australia.