After years of construction and $1.4 billion in spending, the long-awaited Waterview Tunnel has finally opened.

The country's longest tunnel officially opened to traffic at about 12.47am today - five years after construction first began.

A police escort accompanied the first cars to enter the northbound tunnel, travelling from Owairaka to Waterview.

Many motorists, commenting on NZTA's Facebook announcement also expressed their excitement the long-awaited tunnel was finally open.

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A handful had set the alarm early, just for the privilege of going through in its first hours of being open.

However, NZTA Auckland Highway Manager Brett Gliddon advised motorists to avoid rushing through the tunnel in the first few days of its opening, to help ease traffic congestion.

"When it first opens motorists should expect the tunnel to be very busy because we know many people are keen to experience driving it for the first time. The Waterview Connection will be a key part of the Auckland transport system for a long time now and there'll be plenty of opportunity for everyone to use it."

Transport Minister Simon Bridges said the opening marked the most significant change in Auckland's transport system since the Auckland Harbour Bridge opened in 1959.

"This has been a long awaited and eagerly anticipated piece of transport infrastructure envisioned decades ago. It's fantastic that New Zealand's biggest and most ambitious transport infrastructure project is now open to vehicles."

The tunnel was due to open in April, but that was pushed back due to continued testing and commissioning of complex systems for safety purposes.

NZTA did not announce the exact opening time of the 2.4km tunnel in advance, citing safety reasons. It also said it would not give the date or time to avoid queues of motorists.

Bridges said the wider economic benefits of the tunnel were estimated at $430 million, through improved productivity and reduced travel time.

The priority project also created more than 18,000 jobs during the construction period.

"The Waterview Tunnel will transform the way people and freight move around the city, providing more options and a more efficient, resilient and reliable transport system," Bridges said.

"Investing in Auckland's motorway system in this way will reduce the cost of doing business throughout the country and plays a strong role in supporting Auckland's growing population."

In mid July another two community amenities are set to open as part of the project.

Auckland's cycling and walking network will expand further with the opening of the Southwestern Shared Path alongside the motorway, between the southern end of the tunnel and the Maioro St interchange.

The striking Te Whitinga (Hendon footbridge) that spans the motorway will also open to connect New Windsor and Owairaka.

The tunnel completes a key link in the 48km western motorway ring route - a second motorway route through Auckland - and includes a giant motorway interchange at Great North Rd to connect the Southwestern and Northwestern motorways.

The alternative route will ease pressure on Auckland's State Highway 1 and the Harbour Bridge.

NZTA has encouraged the public to plan their journeys and expect it will take some weeks after today's opening, for traffic flows to adjust.