If there is an award that may apply then you need to read it.
You can contact the Fair Work Ombudsman and they will advise you if there is an award or industrial instrument that covers you. If there is you download it and look at the entitlements under the agreement.
Assuming there is one, you can then look at what that says about overtime and pay rates and work out what you should have been paid in accordance to that.
You then secure your position and decide what is best for you to do.
You have time to sue them and I believe you have 3 years.
Do not panic about time you do your research and once you know about the award you can then proceed in your best interests.
Being in WA you may also want to check with the Industrial relations commission WA for the possible award
If your entitlements under the award are better, you can then argue they have underpaid you.
I hope this all makes sense and is of assistance.