The New Democrats are planning to introduce a bill that would break out the tax changes that are part of the budget and fast-track it to ensure they're implemented by July 1.
The changes include cancelling a planned corporate tax cut and hiking taxes for the wealthy — one of the measures the NDP fought for in exchange for their support of the budget.
Duncan said the NDP are reneging on that agreement by refusing to pass the massive 350-page bill in its entirety.
"We're not going to simply pass the things that they wanted and not do the government's budget," he said. "That would be utterly irresponsible. We need a full budget."
The budget isn't just about taxes, he said. The province needs to move ahead with pension fund reforms and other measures to stay on track to eliminate its $15-billion deficit in 2017.
NDP Leader Andrea Horwath's proposal to fast-track the tax changes are "not acceptable," Duncan said.
"We're not going to balance the budget by raising taxes — that's what she's proposing to do," he said. "When is a deal not a deal?"
If the budget doesn't pass before the legislature breaks for the summer, Ontario's credit rating will also be in jeopardy, he warned.
But the NDP say they only agreed to let the budget clear its first major hurdle, so it could be debated and amended if necessary.
Duncan said that would happen sooner if only the NDP would agree to let the budget bill move forward.
He also suggested the NDP are reluctant to give the budget a green light because their union supporters oppose certain measures, such as privatizing some government services.
The Liberals need to realize that they can't always have their way, because it won't get them far in a minority parliament, Horwath said.
"I prefer to put some ideas forward, some offers forward, some way forward, and that's the way I'm going to continue to do things," she said.
"He wants to stamp his feet, poke his fingers in my eyes — that's his choice," she said, referring to Duncan. "I think the people of Ontario deserve a lot better."
The Liberals want to limit debate on the bill, which both the NDP and Progressive Conservatives refuse to support.
The government is trying to shut down a legislative committee that's examining Ontario's troubled air ambulance service, which is under a criminal probe for financial irregularities, said Tory house leader Jim Wilson.
The committee has been grilling government bureaucrats, former Ornge executives and well-connected Liberal insiders to get to the bottom of the scandal, he said. But it only has two hearings left before the legislature rises on June 7.
The Tories and NDP want the committee to sit over the summer, but they need consent from the Liberals too and they're not co-operating, Wilson said.
"There's a very strong possibility — if Dr. (Chris) Mazza continues to call in sick — that the fellow at the centre of all this won't ever have to testify," he said of Ornge's ousted CEO. "The issue may very well die over the summer."