Police are urging drivers to slow down, follow basic road rules and drive to the conditions as a wet Waitangi weekend has already seen a number of serious road accidents around the country.

Four people have been killed in fatal crashes and a high number seriously injured as the long weekend kicked off with some stormy weather conditions.

Two people died in a head on collision at Te Kowhai outside of Hamilton on Friday night.

A man has died after being taken to hospital in serious condition after losing control of his motorbike on Centennial Drive in New Plymouth.

And a 23-year-old man died after he lost control of his car in Manawatu early this morning.

Police say he was taking a bend on Tangimoana Road near Ohakea, and was thrown out of the vehicle.

There have also been a number of serious injuries from car crashes.

A man in his 30s, who was driving a car involved in a crash near Wellsford, north of Auckland, is in a stable condition.

And two people have been injured in Otago, after the car they were in flipped.

The accident happened just before 4am on Weston-Ngapara Road outside Oamaru, the two people were trapped in the vehicle and had to be pulled out.

Meanwhile a serious crash is continuing to hold up motorists on State Highway 2 through the Karangahake Gorge between Waihi and Paeroa.

Three people are in a serious condition following the two-car crash, which happened at about 10.15 this morning.

The road was originally reduced to one lane, causing major delays, and police say it will now be closed for most of the afternoon.

Motorists have been advised to travel via the Kopu-Hikuai Road from Thames or State Highway 29 through the Kaimai Range instead.

"It's an incredibly sad and frustrating start to the long weekend," said Superintendent, Steve Greally, National Road Policing Manager.

"Drivers need to take responsibility every time they get behind the wheel or on their bike.

"Make sure you drive to the weather conditions, if not for yourself, then do it for those who love you and want you to be safe, and for the others who share the road with you."

"Making smart decisions on the road is done by following the basic road safety rules."

He said "Most drivers do a great job, and we congratulate them for making sound decisions. It's the mistakes and poor judgement of the risk takers who put the lives of the innocent in danger."