New Zealand plans to build three tunnels under Auckland’s Waitemata Harbour to ease traffic congestion in its largest city at a cost of as much as NZ$45 billion ($27 billion).

The government has decided to build two three-lane road tunnels, one in each direction, and a separate light-rail tunnel under the harbor, Prime Minister Chris Hipkins said on Sunday. The work, scheduled to start in 2029, would be phased, with construction of the rail tunnel starting after at least the first of the road tunnels is completed, he said.

“This is a bold plan for Auckland’s future that delivers a modern transport network that will connect all parts of the city,” Hipkins said. “Under this proposal the network will become joined up, allowing Aucklanders to travel from the north to the south, east and west on public transport.”

Auckland, home to nearly a third of New Zealand’s five million people, faces extensive peak-hour congestion as the city’s growth strains its motorways. The harbor effectively splits the city in two and the existing bridge, which was opened in 1959, must sometimes be closed due to high winds.

The road tunnels would run nearly 6 kilometers (3.7 miles) under the harbor while the light-rail tunnel would be much longer, extending 21 kilometers from the central city to many of the northern suburbs. It would be built in stages.

Transport Minister David Parker said transport agency Waka Kotahi will report back to Cabinet next year with a detailed plan for how the elements of the harbor tunnels can be phased and funded.

“With a project of this scale we will need to keep an open mind to funding options,” he said.

The announcement comes two months out from a general election. The main opposition National Party said it will look closely at the tunnel proposals if it forms the next government as a second harbor crossing “is a critical part of Auckland’s future.”

But it was dismissive of the plans for more light rail, saying the government had failed to deliver on earlier proposals and has no idea how to fund it.

The Green Party said the light-rail tunnel should be prioritized but slammed the plans for road tunnels.

“Throwing tens of billions of dollars at another six lanes of traffic is not going to solve Auckland’s congestion problem and is grossly irresponsible in the middle of a climate crisis,” it said.