The creation of a spending and revenue watchdog would help prevent wasting billions at eHealth, Ornge and the cancelled gas plants, Horwath said. The office would also provide analysis on government estimates.
Horwath said it would operate much like the federal parliamentary budget office once run by Kevin Page.
"People are upset that their hard-earned dollars have been wasted in the past by the Liberals," said Horwath.
Creating an accountability office would cost roughly $2.5 million as a high estimate, said Horwath.
“No reasonable person would think this is out of the question,” Horwath said at a news conference Wednesday.
When questioned, Horwath said the proposal is not a final offer for the Liberals.
The idea isn't a new one. Finance Minister Charles Sousa was asked about it on CBC's Ontario Today call-in show, and didn't dismiss it.
"It's something that should be of consideration," said Sousa. "We want to be open and transparent."
Speaking Wednesday in Kitchener-Waterloo, Premier Kathleen Wynne called the idea interesting but said she wants a face-to-face discussion with Horwath.
"I don't want to do this in bits and pieces through the media," she said.
The Progressive Conservatives rejected the NDP's call for an accountability officer, saying the province does not need more bureaucrats.
"If you think a government is corrupt and can't be trusted with taxpayer dollars, the answer is not a new bureaucracy, it's a new government," said PC Leader Tim Hudak
With Wynne’s Liberals leading a minority government, they need the NDP to support the budget if a spring election is to be avoided.
Tabled in the legislature last week, the Liberals included a handful of spending measures from the NDP’s wish list, including a promise to cut auto insurance rates and more money for home care.
Hudak has said his party will vote against it.
Wynne said this week on Metro Morning she believes the time to negotiate is over and that Horwath must decide soon whether or not she will support the budget.
Wynne said she’s still open to discussing the NDP’s concerns with Horwath, but said the budget won’t be changed substantively.
"We're not going to start from scratch,” said Wynne.