The Nationals senator John Williams has backed Labor amendments to improve small businesses’ access to justice, in a move that may test Scott Morrison’s hold of the lower house.

The amendments to a Coalition bill are designed to help in competition law cases such as farmers and suppliers taking on Coles and Woolworths in court for misuse of market power.

Williams said the changes, which would allow small businesses to apply upfront to escape paying legal costs even if they lose a case, are “basically a good idea” and predicted they will “pass the Senate without much opposition” – even though they are not official government policy.

The comments spell danger for the government as Labor is intent on weaponising otherwise uncontroversial legislation when parliament resumes in February, passing Senate bills over Liberal opposition in a bid to test the Coalition’s majority in the House of Representatives.

In December Labor and the crossbench combined to add provisions for medical transfers out of offshore detention into a government migration bill, setting a booby trap for Morrison in the lower house in the new year.

The tactic is set to be repeated with the obscure Treasury Laws Amendment (2018 Measures No5) bill.

The Labor senator Doug Cameron has tabled amendments which state that people or organisations suing for damages for alleged restrictive trade practices – including misuse of market power, the new “effects test” designed to protect small business – can apply at any time to be exempted from paying their opposition’s costs.

The court can agree the party will not be liable for costs if the case is reasonable, raises a significant issue that affects others and “the disparity between the financial position” of the parties might deter the smaller party from taking action.

Williams told Guardian Australia the changes were “basically a good idea that bring a balance and greater fairness” in court actions.

“[The changes mean] if I was going to sue Woolworths for trying to screw me down and if I lose then I don’t have to pay costs,” he said.

“If you’re getting put out of business by the big end of town, the last thing you can afford to do is go to court, to lose, and then be hit by costs for your and the defendants’ legal costs.”

Under the changes, small and family businesses can also ask the ombudsman for help in preparing a case for a no-costs order.

Williams predicted the amendments “will pass the Senate without much opposition”, but acknowledged that was his personal view, not government policy.

“When it comes to access to justice, if [Labor’s amendment] is what’s put up it would be supported by most in the chamber … [They support changes] to make it easier and fairer for small business to have the courage to go to court without the fear of losing everything if a costs order is made against them.”

Williams said the effects test – which passed in August 2017 – gave small businesses a remedy “if big businesses are using their market share to undersell, sell cheap and squash their opponents”.

He said it was “ironic” that Labor had opposed the effects test but was now trying to help small business with its access to justice changes.

A spokesman for the assistant treasurer, Stuart Robert, said the bill “remains a priority for the government in the context of its broader legislative agenda” but made no comment on whether Labor’s tactics could delay or impede it.

“Specific programming of legislation is a matter for the leader of the house and Senate leadership team in coordination with cabinet’s parliamentary business committee,” he said.

Malcolm Turnbull agreed to introduce the effects test as part of the Coalition deal with the Nationals.

In late 2017 an insurrection within the Coalition by the Nationals – many of whom threatened to support a Labor and Greens push for a parliamentary inquiry – forced the Turnbull government to call a banking royal commission.

Since then demands have shifted to tackling the market power of big energy companies and Australia’s grocery duopoly.

In December the Tasmanian Nationals senator Steve Martin called for a royal commission into petrol prices and supermarkets.