Mehrtens was found guilty of manufacturing methamphetamine with Waihi man Scott Warren Filer, 48, over a nine-month period between October 2009 and May 2010.
The Crown says that in that time, Mehrtens and Filer cooked between 2kg and 4kg of P, worth between $2 and $4 million.
The pair face maximum sentences of life imprisonment.
Others convicted of drug charges at the High Court trial include Phillip Ly and Hoang Quoc Nguyen, who admitted selling pseudoephedrine, the main ingredient in P.
WANTED
If you see Steven John Mehrtens, call the police or the anonymous CrimeStoppers on 0800 555 111.