Legislation passed parliament on Monday to let employers off the hook for non-compliance with the retirement savings guarantee.

To qualify for the amnesty, employers will have to come forward voluntarily and pay all employee entitlements.

Penalties and administration fees will be waived, allowing payments to be tax deductible in the same way as regular super contributions.

Employers who fail to come forward during the amnesty will face fines of up to double the unpaid entitlements.

Crossbench senators backed the government's bill, with Labor and the Greens voting against it.

Assistant Superannuation Minister Jane Hume expects more than $160 million of super will be returned to workers who would otherwise miss out.

'This policy is all about reuniting hardworking Australians with their super," she said on Monday.

Liberal senator Andrew Bragg said Labor opposed the measures because union-linked industry super funds rejected any changes to the system.

"Voting against it and arguing against it is arguing against the workers that people on the other side of the chamber purport to represent, which is just extraordinary," he told parliament.

About 7000 employers have already come forward and the same amount is expected to own up after the bill's passage.

Australian Council of Trade Unions president Michelle O'Neil said superannuation formed a major part of the wage theft crisis gripping the country.

"This law will recover a tiny fraction of the billions in super estimated stolen since the beginning of the system and will do nothing to change behaviour in the business community," she said.

Unions want the right to superannuation included in the National Employment Standards so repayment can be more easily pursued.

"This is a shameful act by a government which it seems will stop at nothing to cater to employers at the expense of working people," Ms O'Neil said.

Labor frontbencher Jenny McAllister said the amnesty was unfair.

"If you are an employee and you steal from your employer, you have the book thrown at you. The police will come around to your place," she said.

"But, under this government's plan, if you are an employer and you want to steal from an employee, no worries, so long as you say sorry."

Industry Super Australia chief Bernie Dean said one in three Australian workers had been ripped off.

"That members will be reunited with their super is a good thing, but this bill does little to fix the causes of the $6 billion unpaid super scandal," he said.

Australian Associated Press