If you're not sure about staying in Oz, I'd leave your pension right where it is. When the time comes, as Diane mentioned, some countries have agreements whereby time put in elsewhere can be counted toward a local pension.

But even if you stay in Oz, your 5 years may count for something. A friend of mine worked for three years in Switzerland, then returned to the US. A few years later, she retired, and the Swiss actually contacted her about the pension rights she had coming from them - a small sum, maybe 50 Swiss francs a year, but they send it to her without fail.
Cheers,
Bev