The B.C. government has given the go-ahead for BC Hydro to start construction on a third dam on the Peace River, with a delay of six months to try to work out settlements with area aboriginal communities and landowners whose properties will be flooded or cut off.

Energy Minister Bill Bennett said Tuesday an update of the construction costs upheld the $7.9 billion estimate for the Site C dam that was developed in 2010, but the estimate has risen.

Delaying the project six months from its original start date adds inflation and interest costs, and calculating the effect of the provincial sales tax replacing the HST bringing the total to $8.34 billion, and the province is establishing a "project reserve" of $440 million to bring the total estimated cost to $8.77 billion.

The project reserve is in case of unforeseen events such as a rise interest rates during the eight-year construction period.

BC Hydro CEO Jessica McDonald said the six-month delay provides time to work out agreements with Treaty 8 First Nations who have declined settlement offers. The federal-provincial environmental review panel for the project in May, but noted its unavoidable impacts on aboriginal hunting, fishing and trapping rights that are assured by the 1899 treaty.

A group of Peace valley landowners has already started legal action against the project, having refused offers from BC Hydro to buy their land. Aboriginal groups in Alberta, downstream of the project, have also started court action against it.

NDP leader John Horgan called the decision "a $9 billion gamble" and repeated his call for a review by the B.C. Utilities Commission to see if the additional power is going to be needed by the time the dam is operating in 2025.

The BCUC will determine BC Hydro rates in the years ahead, and how much they will go up to pay for the most expensive public construction project in B.C. history. The government plans to mitigate BC Hydro rate impact by reducing the dividend the government takes from the power company's operations each year.

Bennett said a review of alternative clean energy sources, including wind, solar and geothermal power, showed they can't compete on price because they are intermittent sources that would require backup power.