Under the amnesty, employers who put their hand up to admit they haven't paid enough super to employees won't face penalties for doing so, but will have to repay the full amount.

Draft laws setting up the amnesty failed to pass the last parliament, but resulted in about 7000 employers coming forward to admit their wrongs.

Despite the legislation not passing and the amnesty having no legal backing, the Australian Tax Office waived penalties amid the confusion.

Under the original plan the amnesty was due to end in May this year, but will now continue until six months after the new bill is given royal assent.

The government expects another 7000 employers to come forward, with about $160 million owing in super.

Assistant treasurer Michael Sukkar introduced the draft laws to the lower house on Wednesday, and issued a warning for such employers to come forward.

"This is a one-off opportunity to set things right, and going forward the ATO has the tools to spot unpaid super," he said.

The federal government will rake in close to $100 million over four years through the scheme.

However, the Australian Council of Trade Unions said the government is letting "dodgy bosses get away with stealing from workers' super".

ACTU assistant secretary Scott Connolly said the amnesty won't recover the billions of dollars owing since the superannuation system was set up 27 years ago.

"It's a farce set up by a government obsessed with helping business find new ways to exploit workers," he said.

Australian Associated Press