TORONTO -- Ontario's New Democrats want to put the brakes on the government's plans for what they're calling "Lexus lanes" -- their third and last post-budget demand to keep the minority Liberals afloat and avoid an election.

The Liberals must drop plans to allow drivers without passengers to pay a toll to use high-occupancy lanes, at least until they close a tax loophole that will allow corporations to write off HST on entertainment and meals starting in 2015, said NDP Leader Andrea Horwath.

"What they do is basically squeeze out people who are already carpooling for the favour of trying to get people who can pay their way into those lanes," she said. "We don't think that's the right direction to go in."

The government has said it will wait for Metrolinx's final report in June before making a decision about which new fees they'll use to raise $2 billion a year for public transit in the Greater Toronto and Hamilton area.

But the Liberals announced in the May 2 budget that they'll introduce so-called high-occupancy toll lanes (HOT), without saying how much it will cost and inflating previous estimates of how much money it will bring in, she said.

Her party can't support the budget if it adds any new tolls, fees or taxes for individuals while handing tax breaks to corporations, she warned.

"We're actually using this opportunity ... to say, 'We're warning you. This is not a fair, balanced and transparent way to go, and we will not accept that kind of a plan if that's what you decide to bring in the fall," Horwath said.

Finance Minister Charles Sousa, whose budget also promised more high-occupancy vehicle lanes (HOV), said charging tolls could raise about $295 million.

Metrolinx has said revenue for existing HOV lanes is about $25 million a year if they were to charge 30 cents per kilometre.

An earlier report for Metrolinx suggested they could raise up to $250 million annually by 2020, but would require "significant infrastructure expenditures" for lane barriers, vehicle monitoring systems and transaction processing systems.

The report says that converting new high-occupancy toll lanes could cost about $715,000 per kilometre for guardrails and barriers, as well as $650,000 per gantry.

That means millions of dollars will be spent before the lanes generate a dime, the NDP said.

Neither Sousa nor Transportation Minister Glen Murray would say which HOV lanes would be converted or what it may cost.

"We'll provide information in regards to HOVs when we receive all the particulars," Sousa said.

Murray said he doesn't understand what the NDP are after: whether they actually want to drop HOT lanes, raise concerns about them or make it a condition of their support of the budget.

The minority Liberals need the NDP onside to pass the budget and avoid an election. But Horwath is coy about whether her party will support it, saying she wants to meet with Premier Kathleen Wynne first.

But she hinted an election could be avoided, saying she's "not prepared to draw a line in the sand on this."

"And I say that because I believe that the premier will look seriously at these proposals and will have to face the public in terms of whether or not she's prepared to be accountable," Horwath said.

The two are expected to meet Wednesday afternoon.

Wynne said Horwath's extra demands are "a little surprising" given how hard her government worked to put her requests in the budget, such as a cut in auto insurance rates.

"We have to implement the budget now," she said in Kingston, Ont. "We've worked very hard to put in place and even-handed budget."

The Liberals addressed a long list of NDP demands in the budget, but Horwath says she wants more accountability.

Horwath has also asked for a financial accountability office, modelled on the parliamentary budget office in Ottawa, and allowing ombudsman oversight of the health-care sector, such as hospitals and nursing homes.

The Progressive Conservatives are pushing for an election. But they won't say whether they'd give the ombudsman the same powers if they form the next government and have rejected the idea of a financial accountability office.