The $1.4 billion Waterview tunnel will open to traffic in early July, says Transport Minister Simon Bridges.

Bridges said the opening of the roading project, including twin 2.4km-long three-lane tunnels, will mark the biggest milestone in the city's transport network in more than half a century.

"Once open, it will transform the way people and freight move around Auckland, and will represent the biggest change in travel patterns since the opening of the Auckland Harbour Bridge in 1959," Mr Bridges says.

A ceremonial opening on Sunday, June 18 will be followed by public open days to allow people the chance to experience the tunnel up close before opening to traffic.

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"The Waterview Connection is New Zealand's largest ever roading project and is a significant part of the strong investment the Government is putting into Auckland. This investment in the strategic motorway system will help support Auckland's increasing population and economic growth," Bridges said.

"It largely completes the Western Ring Route, a new 48km route linking the west of Auckland, Manukau, the city and the North Shore.

"Together, the Waterview Tunnel and wider Western Ring Route will give more options to people and freight moving around and through Auckland, creating a more efficient, resilient and reliable network," Mr Bridges says.

A series of public open day events will be held in the lead up to the operational opening of the tunnel to traffic. The events will be free but people will need to book their place online by visiting www.nzta.govt.nz/waterview

On February 28, NZTA told the Herald the "Waterview tunnel is on track to open in April 2017", saying there were a number of logistical and safety issues to be worked through.

On March 24, NZTA said the opening had been delayed up to three months due to continuing testing and commissioning of complex systems to ensure the safe operation of the tunnel.

The opening of the tunnel has been hit by delays. It was due to open in April. The New Zealand Transport Agency blamed the delay fault in the software for the jet fan and water extraction pump system which are part of the deluge system.