A number of crashes around Auckland are causing traffic buildups, and heavy delays across the city.

A crash was blocking the right lane south-bound on the Southern Motorway, just before Greenlane, the New Zealand Transport Agency (NZTA) said.

"Expect delays and take extra care," it said on Twitter.

The crash has now been cleared, but delays are still expected.

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Heavy traffic is already building up around Greelane due to the NZ Fitness Expo at the ASB Showgrounds. The NZTA has warned fitness buffs keen to go to the expo to expect delays and plant their journeys.

Meanwhile, a crash on the south-bound lanes of the Northern Motorway after Constellation Dr is also causing delays in both directions, which the NZTA put down to "rubbernecking".

Earlier, a crash on the Harbour Bridge saw traffic backed up to the Victoria Park Tunnel. Congestion was starting to ease, now the truck had been cleared from the city-bound lanes, NZTA said.

An earlier crash on the Southern Motorway, city-bound after the Princes St on-ramp, was also now clear, the NZTA said. But drivers were warned to expect delays.

The series of crashes is causing significant delays to those hitting the road for the first full day of school holidays.