Workers at Holden's Elizabeth plant in Adelaide's northern suburbs have begun voting on a pay proposal that could determine the carmaker's future in Australia.

Holden is looking to save $15 million annually in labour costs to stay viable and has presented workers with a proposal including a three-year wage freeze and cuts to conditions to avoid the plant closing.

The Manufacturing Workers Union has previously said if the 1700 workers reject the offer, the carmaker will close its Elizabeth operations by 2016.

Overnight workers voted at 6:00am (ACST) and there will be another vote after lunchtime.

The union representing Holden workers says it will not be advising its members on how they should vote.

The outcome of the secret ballot is expected to be known by this afternoon.

John Camillo from the Federation of Vehicle Industry Unions says a 'yes' vote will not necessarily guarantee the plant's survival.

"These voters have been told that even if they vote 'yes', [Holden chairman] Mike Devereux is waiting until after the 7th of September in regards to who is elected to govern this country," he said.

"If it's the Coalition, they need to make sure they support the automotive industry and if they don't support the automotive industry this investment could still fall over."

The Federal and State Governments have previously pledged $275 million to the carmaker to keep it in Australia.

Mr Camillo says it has been a tough time for workers and their families.

"No workers want to take a 3 per cent cut but in regards to having a future for the next 10 years you know, these people [will] have a good think in regards to how they vote."

Yesterday, Shadow Treasurer Joe Hockey until after the election to hold detailed talks with the carmaker about options for its future.

Holden workers in Victoria still need to vote on the proposal before a final decision is made.