Metformin false positive for methamphetamine

Hi there, I'm from South Australia. I was recently pulled over for a drug test and the result was positive for methamphetamine. I take Metformin for my Diabetes2 and I believe this was false positive result from the Metformin. I take no other drugs.
I have had a secondary oral test, and the result is the same. Can someone there plus confirm this could be true. My Lawyer is looking for some sort of confirmation.
Many Thanks
P