The woman had to be taken to Waikato Hospital in a serious condition.

A woman was left seriously injured after coming off a motorcycle that struck cattle milling on a dark highway in the Waikato overnight.

The man and woman were riding the Triumph along Hauraki Rd at 1.20am on Tuesday when they came across a cluster of black and white cows.

Fortunately the male rider had just accelerated from an intersection and wasn't travelling at high speeds, Waikato police Senior Sergeant Charles Burgess said.

"He clipped one of the stock on the road and the bike slid sideways from underneath him."

READ MORE:

* Driver on his way to work hits cows in rural Taranaki

* Campervan ripped to pieces after hitting cattle in Taranaki

* Cows dead after collision on rural Waikato road

Burgess said the male rider was uninjured but the pillion passenger needed urgent medical aid.

She was treated at the scene by St John paramedics and transported to Waikato Hospital in a serious condition, accompanied by the man.

The freed cattle were herded back into the nearby paddock, Burgess said.