A massive tunnel being dug beneath Niagara Falls is six months behind schedule, but the Ontario government insists taxpayers won't be on the hook for cost overruns or delays.

"We've got a pretty solid contract to stay on budget," Energy Minister Gerry Phillips told CTV News.

Ontario Power Generation says the tunnel, being built at a cost of about $1 billon, will harness the power of the Niagara River to create 1.6 billion kilowatt hours of electricity a year.

According to the official plan, the 2,000-ton boring machine being used should have drilled about three kilometres by now, but has only drilled about half that distance. The entire project calls for a 10-kilometre tunnel to be dug beneath the city.

Ontario residents won't pay for any delays or cost overruns because of the province's fixed-price arrangement with Austrian tunneling contractor Strabag AG.

If the group completes the project on time and on budget, it will get a bonus. If the tunnel isn't completed according to schedule, the company will take a payment penalty.

New Democrat Party Leader Howard Hampton isn't happy to hear of the delays.

"I think the problem here is the McGuinty government advertised this as a slam dunk when it's not a slam dunk," Hampton said.

The reason for the delay has to do with the fractured rock formations deep below Niagara, says Emad Elsayed, OPG's vice-president of hydroelectric development.

"Going down, you have to go through probably about 10 different types of rock," Elsayed said.

The tunnel walls have been unstable, and boulders the size of cars have fallen onto the massive drilling machine during the process.

The drill, which is 40-feet wide, is the largest of its kind in the world. But it should have an easier time the rest of the way because "now it's going to be essentially the same type of rock for a long distance, and it seems to be in much better shape," Elsayed said.

Elsayed insists it's worth taking the time to do the job right because the project, once complete, will provide power for 100 years.

The original target date for completion was the fall of 2009, but that has now been pushed back to June 2010.

With a report from CTV's Paul Bliss