Commuters are already experiencing problems with the brand new Waterview Tunnel in Auckland hours after it was officially opened.

A breakdown in the tunnel heading northbound was blocking traffic.

After five years of construction and $1.4 billion in spending the long-awaited tunnel was finally opened today.

The country's longest tunnel officially opened to traffic about 12.47am today - but travellers are already experiencing backed up traffic.

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People took to Twitter to show how busy the tunnel was on Sunday afternoon, with GPS map services advising drivers to take alternative routes.

The NZ Transport Agency acknowledged the tunnel was getting busy today, tweeting that it was the "tourist destination of the day".

A car had also broken down in the tunnel.

Traffic maps show the tunnel route in red, meaning heavy traffic.

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."

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.

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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.