An internal Manitoba Hydro report estimates the total cost of the Bipole III transmission line at $4.1 billion.
The report was obtained by the Canadian Taxpayers Federation.
That figure is almost double the $2.2 billion figure projected in 2007.
Critics have argued the provincial government is choosing a longer, more expensive route.
The leader of the opposition, Hugh McFadyen, accused the NDP of hiding the real project cost, leading up to a fall election.
"We have concerns, obviously, about the honesty of the NDP," said McFadyen.
A government spokesperson said the government isn't suppressing cost numbers and said the $2.2 billion figure is still the one they're working with.
A third party is now reviewing the $4.1 billion estimate, but Hydro president Bob Brennan said he doesn't believe the final price tag will be that high.
