"Both women were upset by this news," says a new court judgement which rejects Johnson's appeal against his conviction for conspiring to smuggle meth.
Foiled, the woman left the hat in the car while the pair visited their respective boyfriends in prison booths.
When they went to drive out of the prison car park, however, officers stopped them to search the vehicle.
During a scuffle, prison officers took the hat and found the stashed drugs.
Johnson was later found guilty by a jury of conspiring with others to smuggle meth into prison to supply a fellow inmate.
At sentencing, where he was jailed for two years and three months, a District Court judge said such behaviour undermined rehabilitation programmes and prison discipline.
He challenged his conviction to the Court of Appeal, saying the verdict was "unreasonable", and that the judge had been wrong to conclude the conspiracy involved all 13.92g of meth and not just "a little bit".
Appeal judges, however, found no issues and threw out Johnson's case.