Holiday congestion on the country's main highways is causing delays for commuters heading to see family and friends or just trying to escape the concrete jungle.

It comes as the New Zealand Transport Agency warns of expected delays right through to December 30 across highways around Wellington, Auckland and the Waikato.

Traffic is already heavy north of the capital, with the NZTA reporting holiday congestion between Mana and Paekākāriki.

Motorists are advised of delays in the area.

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Further north on SH1 in Foxton a crash was blocking the road near the intersection of Paranui Rd north of the town.

Delays are also expected there.

In Auckland, NZTA is warning of traffic delays for folks heading south on SH1 through Takanini and Taupiri, and SH2 through Maramarua.

The same is expected north of the city, with delays anticipated through Puhoi, Warkworth and Wellsford until December 30.

In Waikato, SH2 in Maramarua will be closed periodically over the holidays to make travel safer with expected congestion.

All eastbound passing lanes would be closed from 9am to 5pm on Boxing Day and all westbound passing lanes would closed from January 6 to 7 from 10am to 11pm.

Most of the North Island and the top of the South Island could expect a dousing of rain as a complex low-pressure system moves east across the North Island, MetService says.

The weather watchdog has a heavy rain watch in place for the hills and ranges of Hawkes Bay south of Hastings, and the Wairarapa, including the Tararua District.

A watch is also in place about the Marlborough Sounds and Kaikoura Coast.

"Its associated fronts spread south across central New Zealand bringing rain to many places, some heavy, with isolated thunderstorms possible until this evening.'