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The nation's car manufacturing industry gets less total financial support from the government than either the banking or mining industries, a union has argued in a spirited defence of auto making in Australia.

In opposition, the Coalition promised to slash handouts to car makers, and submissions to a subsequent Productivity Commission review on public funding for car manufacturing closed on Wednesday.

The Australian Manufacturing Workers Union, in its submission, said 230,000 jobs were now at risk of being lost.

And it pointed to the Productivity Commission's findings in June that total budgetary assistance to the motor vehicle and parts sector amounted to $620 million.

This compares with $700 million to the mining industry and $914 million to the financial and insurance sector. These figures all included either direct public funding or major tax concessions.

Government assistance to the car industry was less than others, the union's national secretary Paul Bastian said, ''including banking and mining - which are highly profitable and seen to be in the national interest''. Mr Bastian said Australia's auto industry provided $31 to the economy for every dollar of public funding.

He said there would be ''carnage'' wrought on the Australian economy and the national interest if Toyota and Holden next year announced they, like Ford in 2016, were ceasing manufacturing.

But industry policy expert Simon Cowan, a research fellow at the Centre for Independent Studies who has questioned subsidies to the car industry, said there was more than direct assistance for car makers.

''They are also getting significant tariff assistance on imported vehicles,'' he said. These significant taxes on imported cars meant the combined assistance to the local manufacturing of cars was substantial.

The same Productivity Commission report shows manufacturing in Australia received total subsidies and tariff assistance of $7.3 billion in the previous financial year. ''Despite it being 10 per cent of the economy it gets 70 per cent of assistance,'' Mr Cowan said.

Mr Cowan agreed with the union's point that other industries got significant public subsidies.

''But the answer is removing protection for other industries as well,'' he said.