Paying back Stamp Duty - PPOR rates or Investment?

I bought a house as PPOR, first home buyer and got the FHOG and Stamp Duty concession. As I am working away from home at the moment I am probably going to rent it out while I am gone to help pay a bit more of it off rather than have it sit empty.

It will be over the 6mth mark so FHOG is ok, but for stamp duty concession I need to live in it for 12mths - so I will have to pay it back. I was told by OSR that you pay back pro-rata for the amount of time you dont live in it, at the PPOR rates. I have now discovered that for investment properties in QLD the stamp duty is about double, and I was told by the bank that if I rented the house out I would have to pay back the stamp duty at the FULL investment rate pro-rata, not the owner occ. rate? Has anyone been through this, or do you know how it is calculated?