TORONTO - The world's largest rock tunnel boring machine, known as "Big Becky," is scheduled to make a breakthrough deep below Niagara Falls on Friday, four years late and half a billion dollars over budget.

Premier Dalton McGuinty and Ontario Power Generation officials will be on hand for a ceremony when the giant boring machine breaks through to daylight, completing a major phase of a $1.6-billion hydro project.

Big Becky has been working between 90 and 140 metres below the city since the fall of 2006, boring a 10.2-kilometre long, 14.4-metre high tunnel that will eventually be used to redirect water from the world famous falls to hydro generating stations.

The tunnel will be about one and a half times as large in diameter as the Chunnel, the railway tunnel that runs under the English Channel to France, and will generate about 1.6 billion kilowatt hours, enough to power 160,000 homes.

"This is an incredible engineering feat," said Energy Minister Brad Duguid. "It's a tunnel that's four storeys in diameter and may well be the largest tunnel ever dug to this point, anywhere in the world."

Construction crews hit harder-than-expected rock early on in the project, and had to redesign the tunnel to make it safer for workers, which slowed things down and added to cost overruns, said Duguid.

"It had to be re-profiled early on in the project, (mainly) for worker safety, so it did end up costing more than expected, but it's still very good value for money," he said. "It's going to ensure reliable power for the next 100 years, and will cost about $1.6 billion."

OPG's original price estimate for the Niagara tunnel project was $985 million. The Opposition said the Liberal government is to blame for the cost overruns, despite the problems with hard rock encountered by construction crews.

"It's not the first time there's been a tunnel dug in the Niagara region, and certainly I think the geological makeup of the route should have probably have been much better understood before that contract was signed," said Progressive Conservative energy critic John Yakabuski. "We supported the project and thought it was a good idea to increase the amount of hydro (electricity) from Niagara Falls, but unfortunately the government's mismanagement of it has turned it into somewhat of a boondoggle."

Those cost overruns will be added directly to already rising electricity bills for consumers, warned Yakabuski.

"Six hundred million dollars over budget and four years late, so it's hardly a time to be rejoicing," he said.

The amount of Niagara River water available to Canada for power generation exceeds the capability of the existing Sir Adam Beck power canal and diversion tunnels almost two thirds of the time. With the new tunnel, that will be reduced to about 15 per cent of the time, according to OPG.

For the most part, the new tunnel and two existing tunnels follow a route that runs below and to the west of Stanley Avenue under the city of Niagara Falls.

The Niagara tunnel project is expected to be completed and helping to generate electricity by the end of 2013.