The State Opposition says motorists face bill shock when they get renewal notices in the new year because of vehicle licence fee rises.
From January 1, the $72 discount for a private vehicle will be halved.
Opposition Leader Mark McGowan says the move is a cash grab that unfairly targets pensioners and motorists with smaller cars.
He said the fee rise would further disadvantage motorists already hit with a 5 per cent increase for car registration and a 4.1 per cent rise for third party insurance.
People with small cars would be hit hardest because the $36 increase was a bigger proportion of their overall fees.
Their registration renewals would rise up to 75 per cent.
Lansdale pensioner Jim Waddell, who shares a 2004 Hyundai Elantra with his wife, said his registration bill increased 75 per cent from $51 last year to $90.
"To us, as a pensioner couple, $40 is quite considerable," he said.
Treasurer Troy Buswell said the Government would still subsidise motorists $21 million a year and that WA had the nation's lowest combined vehicle registration and third party insurance premiums.
He said changes to the private vehicle registration subsidy was announced in the August State Budget and was one of many decisions in the plan to help return the State's AAA credit rating.