I believe that the best chance of getting interest is to calculate it in the same way that the applicable court would.
(@Rod or any other professionals - Please correct me if the following information is incorrect.)
It is my understanding, that the applicable rates are pretty much the same across the various courts and jurisdictions, being
Pre-judjment rate = RBA Rate + 2%, and
Post-judjment rate = RBA Rate + 4%.
The rates are updated every six months based on the last rate published by the RBA prior to 1 January and 1 July each year and each court usually maintains a full historical rate table on their website.
For historical claims such as this, you should calculate the interest for for each six month block based on the applicable historical pre-judjment rate for that period.
Additionally, I'm pretty sure that courts advise that simple interest should be used, not compound interest. As I understand it, courts don't charge "interest on interest" (compound), they only apply simple interest.
If you:
1. calculate interest according to how the applicable court would calculate it, and
2. you explain exactly how you've calculated the interest, and
3. you provide a reference to the applicable interest rate table that you used,
then you greatly increase the chance that the other party will concede and pay the interest exactly as you have calculated it. If you use any other method, such as compound interest which would greatly increase the amount payable, then you risk other party refusing to pay interest at all. If the matter then proceeds to court, then it will be up to the court to decide if interest is even payable in the first place, and if so, the court will most likely calculate the interest according to their normal procedures as described above anyway.
It's best not to appear greedy to the other party. In your circumstances, you have a valid case for claiming interest, but if you over do it, you would likely have to fight for it and I believe you would be somewhat disappointd with the result.
From personal experience, I can tell you that I followed this approach recently in a matter with an energy retailer. The matter involved breaches of the ACL, so I calculated the interest using the rates applicable to the Federal Court. The company paid all amounts owing plus the all interest without any questions.