A coalition of Indigenous leadership, environmental advocates and others have launched a petition demanding the federal government release an updated cost estimate for construction of the Trans Mountain pipeline expansion.

The coalition includes the Union of B.C. Indian Chiefs, West Coast Environmental Law, environmental group Stand.earth and citizen's group Dogwood. They announced their petition at a news conference on Tuesday in Vancouver.

They say the federal government has "gone dark" around the project's finances and that the publicly available costing data is "stale."

Alexandra Woodworth, campaign manager with Dogwood, said Canadians need to think like shareholders on the project.

"What do shareholders expect when it comes to their investments? Two key things are regular financial reporting and a clear return on investment and how that's going to come about," she said.

"Just how much is Trans Mountain going to cost us, here, now, in December 2019?"

The expansion project would twin the existing 1,150-kilometre existing pipeline, nearly tripling its capacity to move oil from Alberta to coastal B.C., and then to markets in Asia.

The last published information, according to the coalition, estimated it could cost between $7.4 billion and $9.3 billion to twin the existing line.

The $9.3 billion figure came from Kinder Morgan in August 2018 as shareholders were getting ready to vote on the sale of the pipeline to the Canadian government.

In 2013 the company estimated the expansion would cost about $5.4 billion.

Eugene Kung, a lawyer with West Coast Environmental Law, said since Canadians became the owners of Trans Mountain, "It is in fact harder to get up-to-date, accurate, financial information about this project than when it was owned by Kinder Morgan."

He said that's due to the fact that Kinder Morgan is legally required to file quarterly and annual updates to shareholders.

Kung said citizens have the right to a full disclosure of the costs "to assess whether it is in fact going to be economic."

Work on the Trans Mountain pipeline expansion underway in Parkland County outside Edmonton. (Travis McEwan/CBC)

Chief Kukpi7 Judy Wilson, secretary treasurer of the Union of B.C. Indian Chiefs, said there's a "total lack of transparency" around the accounting on Trans Mountain.

She said the public needs to see the numbers, "so we can have a full discussion around the billions of public dollars spent on oil infrastructure during a time when Indigenous communities lack safe drinking water and critical work on reduction of carbon emissions is needed for a climate safe future."

In an emailed statement, a Crown corporation spokesperson said Trans Mountain can't predict when it will have an updated cost estimate for construction, but aims to do so "as soon as possible in the new year."

The statement said Trans Mountain is still going through the process for permitting and regulatory approvals that have an impact on the project schedule.

"Once we have greater clarity on those approvals we will refine and adjust our schedule and be in a better position to release project costs."