A crash is causing a major headache on a new highway leading to Auckland Airport. Traffic is so bad people trying to get to the airport are being told to use a different route.

The new underpass on SH20A opened to traffic this morning and within 60 minutes was bumper-to-bumper.

But by 8.45am a crash at a roundabout had led to big delays for people heading to the domestic and international terminals.

The New Zealand Transport Agency advised people to use the southern Puhinui Rd entrance to the airport to avoid getting stuck in queues.

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The newly upgraded intersection on the main road to the city's airport opened before 6am.

By 6.15am slow-moving traffic had filled both lanes of the new underpass and authorities warned those heading to catch a plane this morning to allow extra time and expect delays.

The traffic continued to be heavy throughout rush hour. Authorities said it was slow-moving heading to the airport between the Southwestern Motorway and the Landing Dr roundabout.

An NZTA spokeswoman said the heavy traffic flows could be put down to people getting used to the new road layout and looking at the new route.

NZ Transport Agency's senior manager project delivery Chris Hunt said separating the two sets of traffic would improve flows and make travel through the area safer and more reliable.

"A number of intersections along the route have also been improved, which will help connect the local community and make it safer for pedestrians and cyclists, especially the many school children using the area each day," he said.

The highway is considered the gateway to Auckland and forms a key link between the Western Ring Route, the airport, the port and greater Auckland.

The agency was continuing to work with Auckland Transport and Auckland Airport on a range of short-, medium- and long-term measures to reduce congestion in the areas immediately surrounding the airport.