Watch: Picnickers smashed by massive wave in Coromandel after volcanic eruption in Tonga

A group of picnickers in the Coromandel Peninsula had an unpleasant surprise on Sunday when a huge wave smashed over the sea wall and into them. 

The unusually large waves came after an underwater volcano off Tonga erupted, prompting tsunami warnings for several South Pacific island nations including New Zealand. 

New Zealand's emergency management agency (NEMA) issued an advisory on tsunami activity for its north and east coasts with the areas expected to experience strong and unusual currents, and unpredictable surges at the shore.

Footage taken at Buffalo Beach in Whitianga shows a group of people sitting at a wooden table a few metres back from the beach. The group is relaxing when a large wave smashes over the sea wall and into them. 

Aucklander Clash Richardson, who took the video, told Newshub the waves were huge.  

"We grabbed some takeaways and went to sit down there…and we were taking some video because of how big the waves were. A few sets [of waves] were coming in closer and I just panned around and then it hit. I didn't even realise those people were sitting there and it just smashed them.

"They were squealing and jumping up and getting out of the way. 

"There were probably at least 100 people down there last night watching the action and for it to come in that far and be breaking on the seawall and splashing that far is not that common."

It wasn't the only beach with unusually large waves. Matapouri in Northland had large wave surges on Sunday night as well.