Bus lanes are the biggest thing missing from Auckland's new Waterview motorway tunnels, Auckland's mayor says.

The 2.7-kilometre twin tunnels between the South-Western and North-Western Motorways will be the longest in New Zealand and will complete the Western Ring Route.

Auckland mayor Phil Goff said they'll provide a valuable connection between west and south Auckland. But he says Auckland's congestion problems won't be solved until dedicated bus lanes are built on transport corridors like Waterview.

"It's really important that we increase our bus lanes. There should always have been a bus lane on major motorways - probably including this one actually."

Mr Goff said the construction of light rail to the airport also needs to start as soon as possible.

Yesterday Prime Minister Bill English was quizzed about his government's commitment to rail, during the ribbon-cutting for the new motorway tunnels.

Mr English said the government has already committed to co-fund the City Rail Link which will double rail capacity. It involves a $24 billion-dollar 10-year plan with Auckland Council, and long-term plans are also being made.

"Almost all of that is funded and of course the discussion has now moved on to what's going to be next after that."

Thousands of Aucklanders have had a sneak peek inside the Waterview tunnel, which is about two and a half times the length of the city's Harbour Bridge.

Those who snapped up tickets to the first of five public open days yesterday afternoon were impressed with what they saw.

One visitor to the tunnel told Newstalk ZB they thought it looked "absolutely wonderful" and another wished it had been open two decades ago. Others were just happy they'd had a run through before they made the trip by car.

NZTA Auckland highways manager Brett Gliddon says the tunnel was cut by Alice, one of the largest tunnel boring machines ever created.

He said the Waterview interchange was built from Dennis, a gantry named after project worker Dennis who died from cancer.

"How we placed the beams to reduce disruption to the motorway underneath was really innovative and Alice had a few little fancy things too - how it laid the culvert and the likes - which was really good to see."

Further public open days are planned this month, before the tunnels open to motorists early next month.