Follow the deal link to order a FREE Belong Mobile SIM. While you fill in the form, remember to enter a referral code in the Promo code section (get a referral code from OzBargain's randomiser below). When you activate the SIM, you will get a FREE $20 credit to use on any mobile plans (e.g. free 2 months on 1GB plan).
I can also confirm that you can transfer your number from Boost or Telstra to Belong directly.
Traditionally, you had to buy a SIM starter pack to get the $20 referral but with this method, it will cost you nothing.
For everyone's piece of mind, I sent a support request regarding referral credit and this is the reply I got
THE BELONG TEAM
Hi. Greetings from Belong Mobile! We received your request for assistance about the billing of your account. Let me help you. Upon checking, your account has a remaining credit of $10. The $20 referral credit has been applied already, and $10 of it was deducted today for your service from today until 19 April 2019. I assure you that your account is up to date, and for the next invoice the remaining $10 credit will be utilized. Hopefully, this clarifies your confusion. Regards, John @ Belong Mobile