Easter traffic is starting to build as holidaymakers head out of the city for the long weekend.

And the NZ Transport Agency says it is shaping up to be the busiest Easter ever on New Zealand roads.

State Highway 1 north of Auckland is now congested through the Johnstones Hill tunnels and the agency is telling motorists to consider using SH16 to Wellsford instead.

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NZ Transport Agency tells Easter travellers to prepare

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The southern motorway is heavily congested southbound from the city to Takanini following earlier incidents - drivers are being told to allow extra time.

A truck-trailer has also crashed and is partly blocking the same highway south of Whangarei. A temporary road closure is in place and the NZTA is warning motorists to expect delays.

Meanwhile, all roads are now open around the South Island but drivers are being warned to expect possible delays through Franz Josef on SH6.

Transport for Christchurch warns to expect heavier traffic flows than usual this evening and said particular hotspots include SH1 between Christchurch and Amberley and SH1 between Christchurch and Ashburton.

The NZTA's road safety director Ernst Zollner said that 279,000 more vehicles on the country's roads compared with a year ago increased the potential risks for drivers.

"We all need to approach the long weekend with a 'safety first' mindset to minimise the chances of making mistakes that can cost lives and cause serious injuries," he said.

Holiday periods also meant motorists were more likely to spend many hours behind the wheel and he warned fatigue could be deadly while driving, especially travelling unfamiliar roads.

"We all make mistakes some times, but mistakes on the road can have serious or even deadly consequences, so ensure your car is road fit, keep your speeds down, drive to the conditions and stay alert," Mr Zollner said.

Last year there was one fatal crash and 101 injury crashes that left 29 people seriously injured during the Easter holiday period last year. Nearly all of those crashes occurred on the open road and 40 per cent were the result of a driver losing control or running off the road.

Mr Zollner said one of the simplest measures to keep safe was to reduce speed.