Major changes to Ontario's auto insurance regulations in 2010 "dramatically" reduced benefits for drivers and cut accident payouts in half, said Horwath.
"In 2011, the value of statutory accident payouts fell by just under $2 billion, an astonishing 50 per cent reduction from 2010, but that same year Ontario's auto insurance rates still increased by five per cent," she told reporters.
If the industry won't cut premiums voluntarily, the government should mandate it through the provincial regulator, the Financial Services Commission, added Horwath.
"The bottom line is if they're not prepared to change it, then yes, government will step in and say 'You must reduce your rates by 15 per cent within the upcoming year,'" she said.
The NDP calculated a 15 per cent reduction in premiums would save the average Ontario driver about $226 a year.
The Insurance Bureau of Canada said 95 to 97 per cent of every dollar raised in premiums paid for claims in 2011, with the auto insurance sector showing a profit of $233 million compared with losses of $1.76 billion in 2010 and $824 million in 2009.
"We are seeing improvements, yes," said Pete Karageorgos, IBC's manager of consumer and industry relations.
"We in Ontario still have some of the highest claims payouts across the country."
Elections Ontario returns show the Insurance Bureau donated more than $70,000 to candidates in the Ontario Liberal leadership contest, including $25,000 each to Wynne and former Windsor-West MPP Sandra Pupatello, who finished second.
The minority Liberal government will need the help of at least one opposition party if it hopes to avoid an election this year, and the New Democrats have a series of issues they want dealt with if Wynne wants their support.
Horwath also wants $30 million in funding to eliminate home care waiting lists and institute a five-day guarantee for seniors who need health services at home.
The government said 90 per cent of seniors who need home care get it within nine days of a referral by a doctor or social worker.
Last week, Horwath asked Wynne to close $1.3 billion in corporate tax loopholes, spend $200 million to create jobs for youth, and call a public inquiry into cancelled gas plants in Oakville and Mississauga, which cost taxpayers at least $230 million.
The NDP is not dictating what must be in the upcoming budget, but it wants to see movement on its key concerns this spring, said Horwath.
"None of the movement can happen, in my opinion, if we don't have a real commitment to the accountability around the gas plants," she said.
"I'm not saying these are the specific details that have to be in a budget, but I am saying we have a session upcoming and these things are very achievable and they need to be part of the government's plan for this session."
Wynne said last week that she wouldn't want to spend millions of dollars on a public inquiry, but she did not rule out a Progressive Conservative request to have a legislative committee study the cancelled gas plants.
The Tories, meanwhile, released a web-only video attack ad against Wynne on Monday, saying they were "worried" because the premier-designate has said she wants to continue the legacy of outgoing Premier Dalton McGuinty.
Wynne must commit to reducing government spending and eliminating the $11.9-billion deficit sooner than 2017-18 if she hopes for Tory support in the legislature, said PC finance critic Peter Shurman.
"We're putting her on notice: we expect a plan from premier-designate Wynne to address the jobs crisis and a plan to address the financial crisis that this province is in," said Shurman.
"Given Wynne's history, Ontarians should brace themselves for higher debt, greater unemployment and higher taxes."
Wynne and her new cabinet will be sworn in next Monday, and the legislature will resume sitting Feb. 19 with a speech from the throne opening the spring session.