The Community Action for Windsor Bridge group's leaders say they will be left bankrupt or forced to sell their homes and businesses if they are slapped with cost orders for a case they believe is in the public interest.

The group took the RMS and the Planning Minister to court to challenge plans to demolish the Windsor Bridge, build a new bridge about 35 metres down stream and put a road through the state-heritage listed Thompson Square, laid by Governor Lachlan Macquarie in 1811.

The eight founding members - among them a store manager, a retired high school principal and an antiques shop owner - had pro bono legal representation and agreed to shoulder any costs awarded against them when they lost the case.

But the toll, they said, would be significant. And it could get worse.

A spokesperson for planning minister Rob Stokes said he was "considering" whether to seek costs.

Megan Wood, who would likely have to sell her small business and property to cover her portion of the costs, said the RMS was trying to make an example of the group for challenging the decision.

"No other community group will ever try to fight the ridiculous bloody mindlessness of the RMS if they get up on," Ms Wood said.

"Everybody will be too scared to ever do it again. It's just terrible."

Another member, Harry Terry, said the group had not been deterred from their campaign and they had nothing left to lose.

"I would rather go to jail than give up my home which would be the two choices I would guess I would have," Mr Terry said.

"I do not have that money. I am a retired school principal and I certainly do not have anywhere near that in my bank account."

The group is placing their hope in a Land and Environment Court regulation that allows the court to reserve making an order for the payment of costs if the proceedings have been brought in the public interest.

Roads Minister Duncan Gay said the "small group" had significantly delayed the construction of the new bridge.

"The court case and subsequent delays in building the new Windsor Bridge has cost the taxpayers of NSW hundreds of thousands of dollars," Mr Gay said. "In addition, the benefits of a new bridge have been delayed at least 18 months."

While both sides agree that a new or upgraded bridge is needed, those against the government's plan have argued that a bypass around Windsor is a better option as it would preserve the heritage of the area.

The action group's members have been occupying Thompson Square, one of the oldest public squares in Australia, 24 hours a day for more than 830 days in protest.

Australian novelist Kate Grenville, who penned the bestselling The Secret River set in the Hawkesbury, has also weighed into the debate, issuing a plea to preserve the character of the square.

The costs hearing with the RMS will go before court on Tuesday.

Minister Gay said the plan for the bridge had not changed, and the project would go ahead as approved.

While the legal proceedings were underway, the funds reserved for the Windsor Bridge project were used to build another controversial bridge, the Albert "Tibby" Cotter Walkway at Moore Park.

UPDATE: In a statement on Tuesday, Parliamentary Secretary for Western Sydney and Member for Castle Hill Ray Williams said the government would not be pursuing costs against the action group.

"Based on the latest advice we have received from RMS, we will no longer be pursuing the Community Action for Windsor Bridge group for the hundreds of thousands of taxpayer's dollars spent as a result of their court case and subsequent delays in building the new Windsor Bridge which the majority of the community wants to see completed," Mr Williams said.

"Community Action for Windsor Bridge is a not-for-profit community organisation. We are advised that it is unlikely that there would be any significant recovery of costs in this particular case. Given this, we are not prepared to spend any more taxpayer's money on further proceedings."