About 100 people are believed to have been on or around Whakaari/White Island when it erupted, the Prime Minister says.
For live updates of the White Island eruption, click here
Jacinda Ardern addressed the eruption at her weekly post-Cabinet meeting.
"All our thoughts are with those affected at this stage," Ardern said.
Emotional families of those affected are gathering at the Whakatane wharf.
They've been told only one family member per person is allowed beyond the police cordon.
The island began erupting about 2.15pm. Images from the crater rim just minutes earlier appear to show people walking close to the crater.
Many of the people visiting White Island today are understood to be from the cruise ship Ovation of the Seas, which was berthed at the Port of Tauranga this morning.
Port company chief executive Mark Cairns says he understands the majority of those injured in the eruption are from the ship, the world's largest cruise vessel.
NZME reporter Kate Shanks is at the Whakatane wharf, and said ambulance after ambulance is arriving, and boats moored along the wharf have been asked to move.
She said there were emotional scenes as family gather and wait for news.
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A local boatie, who didn't want to be named, said he was on the water when the eruption hit and saw one of the White Island Tours boats was already heading in to shore when Whakaari exploded.
The boat drove at high speed to get to the wharf and dropped off its passengers before heading back at speed towards the island.
A shelter was installed on the volcanic island in 2016 in case of an unexpected eruption like today's.
The 2.4 tonne shipping container has been placed by the Defence Force on an old mining site to provide a natural protective barrier. It's not yet clear whether the shelter was used today.
A man whose Facebook profile says he works at White Island Tours has posted a photo of the island erupting, saying: "White Island just erupted as we left, we have our workmates and a tour still on the island, I hope they okay."
He later said the boat he was on was returning to the island: "We have people in critical condition [to] help."
Tourism Bay of Plenty spokeswoman said a Volcanic Helicopters chopper and a boat have still not been accounted for.
Several rescue helicopters are responding and an emergency operation centre has urgently been set up at Whakatane Hospital.
The National Emergency Management Agency has issued a national warning and is saying it's hazardous in the vicinity of the volcano after the "moderate" eruption.
Civil Defence is telling people to stay out of the designated restricted zones, where ashfall may be a problem, and is warning people who live close to the island to stay indoors.
Long-term resident Maree Reeve said it was "possibly the worst eruption she had even seen from the island".
Dan Harvey was out fishing with some friends 40km away from the island when he saw dark smoke erupting from it before 2.15pm.
"I looked over and saw a burst of steam coming up. There was nothing above the island at that time. It was just clear blue sky. It was unusual to see it go from nothing to steam erupting out of it.
"It was almost like a mushroom cloud ... the steam was expanding and getting bigger.
"I knew it was odd because there was nothing above it. Often it's doing this small continuous release of steam. But it wasn't doing anything then all of a sudden this massive amount of cloud comes out."
Harvey said it's still erupting steam but it was nothing like it was when it first erupted.
He added that it happened so quickly and fast that if there were anyone on the island it wouldn't be good.
CEO of Tourism and of Plenty Kristin Dunne, said it was a "shocking" event.
"Our concern is for any visitors that have been on the island and how we can assist when they return to the mainland," she said.
The company is working closely with police and civil defence to establish more information, she said.