More than three-quarters of foreign workers employed under the Government's new visa system would be free to work without any labour market testing because of exemptions under Free Trade Agreements, according to Labor.

Labour market testing, a key element of the Turnbull Government's 457 visa revamp, requires employers to advertise jobs before filling them with foreign workers, as evidence of worker shortages.

But Opposition Leader Bill Shorten has told Lateline, under Free Trade Agreements already signed and those the Government hopes to sign, 77 per cent of 457 workers could get a job in Australia without their employers having to search for an Australian worker first.

"Let's call Turnbull's changes what they are, they are a con job, not a crackdown," he told Lateline.

"We won't be signing future trade agreements that exclude labour market testing. Turnbull will."

Importantly, the Opposition's claims refer to agreements that are yet to be signed, with countries like India, Britain and the European Union.

Workers from India and the UK, made up 42 per cent of workers employed in Australia under 457 visas in 2015/16.

A spokesman for the Prime Minister said Mr Shorten's claim was purely hypothetical.

"It is telling that the overwhelming majority of cases the Opposition Leader is referring to are from Free Trade Agreements that do not exist. As usual, he's making stuff up," the spokesman said.

Labor has long called for labour market testing.

It also supported each of the Free Trade Agreements the Government has already signed.

Mr Shorten said there should be more rigorous independent labour market testing.

"Where this Government falls down on the job is … they're not going to have sufficient or expansive independent labour market testing. They're going to rely on employers to do it," he said.