I bought a playstation 3 in South Africa and have moved to Australia and I can get any game or DVD to play on it that I have bought in Australia why?

I think you wanted to write that you CAN'T watch DVDs or play games bought in Australia on your PS3.
The reason is the region code of your playstation. It only can play Region Code 2 DVDs and games. In Australia you only can buy Region Code 4 media.
You need to buy your media from south africa or Europe. Some blueray-discs are region code free. You can check this on: http://www.blurayregioncodes.com/
As far as I know it's not possible at the moment to change the region code of the play station.

The PS3 is region free for all games, meaning that you can buy a PS3 from all over the world and have it play the game you buy from any country or region. But the console is region locked for DLCs. It means if you bought a PS3 from the US, you'll have to get the DLCs from a North American PSN account. PSN codes on your PS3 are locked to the country that it's bought from and it needs to match the PSN account.
And the PS3 is region locked for Blu-rays and DVDs. Users are allowed to change the region code for playback of DVDs on certain models and there's no such option for changing the region code for Blu-rays. 1100s model or higher allows users to change region code for DVDs but not 1000s models. Below are the lists of region codes of Blu-ray and DVD for your reference.
You can remove the region code on the disc by creating a region-free copy of the disc. Or you can get a region-free video player on your computer. After all, there are many region-free software tools you can find online.