The opening of New Zealand's largest roading tunnel project has been delayed up to three months.

The $1.4 billion Waterview Connection motorway project in Auckland might not open until winter starts.

The NZ Transport Agency has just issued a statement saying the expected opening next month would not now be possible and the project might not now open for two or three months.

"Testing and commissioning of the complex systems that will ensure the safe operation of the Waterview Tunnel are continuing and will not be completed in time for an April opening," it said.

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"The Transport Agency and Well-Connected Alliance are working towards opening the tunnel within the next two to three months, with precise timeframes dependent on the testing and commissioning work currently under way."

Brett Gliddon, the agency's Auckland highway manager, said the testing phase of the job was critical.

Fletcher Construction has a big role in the project as part of the alliance building it.

The twin 2.5km tunnels and 2.5km of motorway is to provide a second route through Auckland, bypassing the city centre.

The Herald reported earlier this month that the new motorway was set to open in April, most likely the weekend of April 8 and 9.

Gliddon said at the time that the only remaining works involved polishing road surfacing, line marking, signage and testing all the electrical tunnel equipment before the route can be used.

When finished the connection would provide the "missing link" between the Southwestern Motorway and SH16. The new route will provide faster more direct transport options and prevent total gridlock if an incident blocks one motorway, he said.

Existing infrastructure is feeling the strain of Auckland's burgeoning population because since 2013 Auckland has grown by 121,000 people and there are 44,000 more vehicles on Auckland roads than this time last year.

Gliddon said earlier this month that the benefits to Aucklanders would be "huge".

People who live in the north and west will have an alternative route through the city, there will be a full motorway connection to the airport and motorists can bypass the CBD on the Western Ring Route.