Traffic is building across the country and travellers in some areas are being warned of potential delays as many take to the road before the long weekend.

Waka Kotahi NZ Transport Agency has advised road users the right southbound lane just adjacent to the State Highway 1 Takanini off-ramp has been blocked by a crash.

Emergency services and response crews are on-site to clear the incident as soon as possible and the agency said people should pass the scene with care and be prepared for delays.

State Highway 1 is also busy at the Greenlane Interchange and Northcote Rd.

Things are more settled in Wellington, but traffic is building.

In Christchurch, the roads are also filling up as Kiwis start their long weekend.

Mixed weather is expected over the Anzac Day long weekend, which is forecast to be filled with on-again, off-again showers.

It starts with most parts of the country expecting showers today which will then clear into fine sunny skies on Saturday before the rain sets in again on Sunday and Monday, forecaster MetService says.

Sunday's rain will be heavy in some regions, while a cold front will also move north from the bottom of the country, bringing cooler temperatures to the South Island into next week.

Auckland can expect more rain this morning before the weather clears this afternoon for a top of 21C, MetService says.

Waka Kotahi NZTA has warned motorists to be prepared for wet weather and surface water across Auckland's motorway network today.

It has also warned Aucklanders heading south today out of the city towards Coromandel and other holiday spots that traffic delays are expected between 10.30am and 7.30pm.

Things are expected to brighten up by Saturday, however, which is tipped to be fine and sunny with a high of 19C before showers blow in again on Sunday and Monday for tops of 20C.

Those holidaymakers heading to Northland can expect an identical weather pattern of morning showers today, with a top of 22C that then clears in the afternoon and tomorrow for a high of 19C before showers set in again on Sunday for a maximum of 20C.

Holidaymakers heading to the Coromandel and Tauranga-Bay of Plenty this afternoon also need to be careful on the roads, meanwhile.

Both regions are currently being drenched by heavy rainfall and severe thunderstorm watches are in place until 5pm today.

Flooding has closed SH25 in the Coromandel, cutting local communities off from Whitianga.

In Tauranga, the rain should eventually clear for a fine Saturday and top of 20C before showers move in again on Sunday and Monday for highs of 21C.

Further south and east, a heavy rain watch is also in place for Gisborne through until 6pm.

Wellingtonians can expect clearer skies than those in the north, according to MetService.

Showers are possible in the city this afternoon along with a high of 20C.

Saturday should then clear into fine weather - albeit with a chilly high of 16C - while Sunday and Monday will also largely be fine and have highs of 19C and 17C.

In the South Island, Christchurch is bracing under morning showers, but that is expected to clear by the afternoon for a high of 16C.

Saturday should then remain fine and clear for a high of 18C, before showers return on Sunday with a high of 21C.

Later in the week on Wednesday, the thermometer is expected to fall to 15C in the city.

Dunedin is also experiencing morning showers today for a high of 14C. Rain eases on Saturday but clouds will still hang round, with a high of 18C, MetService says.

Sunday will then be rainy again with a high of 16C.