If you're planning a trip to the UK or Europe this is a great alternative to buying a travel Sim card.
GiffGaff (UK mobile provider) will send a mobile Sim card for free to Australia. You will have to top it up in order to activate it.
All-inclusive prepaid plans can be bought for 8 GBP (with 5GB data) or 10 GBP (with 20GB data). More details here https://www.giffgaff.com/sim-only-deals
Plans include roaming in the EU, making it handy for a trip through the UK and Europe.
And if you for some reason need a UK phone number in Australia you can also activate the card in Australia by loading it up with 10GBP credit, and use that credit to buy a prepaid plan when/if you get to the UK. To my understanding the credit doesn't expire if you don't use it.
Sign up with a referral link (Ozbargain has some for Giffgaff if you click through below) and they give you 5 GBP credit for free when you activate your Sim card.
Ah didn't know there were ref links here. I ended up ordering directly a couple of weeks ago and it's already delivered.