Ontario's auditor general says she'll look at how reasonable the province's cost estimates were for Hamilton light-rail transit (LRT) as part of an upcoming audit of Metrolinx.

A key construction union is also investigating the government's numbers, and says its preliminary investigation shows the province has been misled.

Auditor General Bonnie Lysyk said in a letter to Ontario NDP leader Andrea Horwath Thursday that she already started a value-for-money audit of Metrolinx governance and operations earlier this month.

"As part of this audit, we will be looking at the reasonableness of the cost estimates for rapid transit projects, including the Hamilton LRT," she said.

Lysyk was responding to a request from Horwath, who represents Hamilton Centre. The province cancelled an LRT project earlier this week that runs 14 kilometres through three NDP ridings, including Horwath's. The province will still invest $1 billion in Hamilton, Transportation Minister Caroline Mulroney said, but a task force will decide how it's spent.

Horwath wrote Lysyk on Wednesday asking her to look into the Ministry of Transportation estimate that Hamilton LRT would cost $5.5 billion in capital, operating and maintenance over 30 years.

Andrea Horwath says Ontarians deserve to see the full report that led to the cancellation of Hamilton LRT. (Samantha Craggs/CBC)

Mulroney said the estimate was the reason her government cancelled LRT. The ministry hired a third-party consultant, she said, after "proponents in the market" alerted her that the Hamilton project was over budget.

Horwath wants Lysyk to investigate why the cost-per-kilometre is higher for Hamilton's LRT than the Hurontario and Finch West lines.

"The minister is refusing to disclose the third-party consultant's report that the premier cites as validation for his figures," Horwath told Lysyk, "so the public has no way of independently assessing them."

The Laborers' International Union of North America (LiUNA) announced earlier this week its investment arm would do its own review of the numbers. The union said in a statement Friday that preliminary figures show the province's numbers were inflated.

The union did not release its analysis.

"We do know without question that the advice given to the province was completely inaccurate," the union said. "Why, we do not know, and we urge the province to start asking this important question."

Union officials met with Mayor Fred Eisenberger Friday morning, and are also working with the Hamilton Chamber of Commerce to try to revive the project.

LiUNA has had the ear of Premier Doug Ford in the past. The union backed Ford last year, and Ford has attended numerous LiUNA events and rallies.

Some 5,000 workers from LiUNA Local 837 were expected to work on construction of the 14-kilometre line, which was scheduled to be in operation in 2024.

LiUNA is also building two 30-storey towers, or 529 units, at King and Hughson, which is on the LRT line. It also finished a 20-storey student residence on nearby James Street North last year.

A citizen group called the Hamilton Transit Alliance also released a statement Friday, saying cancelling LRT "will prevent Hamilton from realizing the many social, economic and environmental benefits that reliable public transit brings to a community."

Not everyone was in favour of LRT. Judi Partidge, Ward 15 councillor for Flamborough/Waterdown, released a statement of her own this week. LRT, she said, "did not have unanimous support around the council table."

"There is no question that the LRT project would have increased your taxes, which directly affects affordability and everyone's ability to stay in their homes and in the communities they love," she said.

Partridge says it's time for the city to move on and focus on the $1 billion investment.