The union representing Melbourne's public transport workers has been granted permission to hold a ballot on industrial action.

The Rail, Tram and Bus Union said the decision by the Fair Work Commission could result in a strike by early next month if no agreement is reached with train operator Metro.

The previous agreement covering operations staff expired on June 30.

The union is seeking an 18 per cent pay rise over three years for Metro operations staff.

State secretary Luba Grigorovitch said union members would now vote on a range of measures.

"If we can't reach agreement on a number of critical conditions that our members have fought to defend for decades, then we will have no choice but to take the action," she said.

"One of our protected action items would be to not enforce myki validation for the day, so effectively a free travel day for the public."

Ms Grigorovitch said the union's objective was to minimise the impact on the travelling public.

"The aim is to have Metro understand the value of the workers who keep the network running and to get serious with their negotiations," she said.