Tims said it wasn't safe to go and look for those who are unaccounted for yet.
"We know the urgency to get back to [the] island," he said.
"At this stage it is too dangerous for police and rescue to go onto the island ... the island is currently covered in ash and volcanic material."
St John Director of Operations Norma Lane said 11 helicopters and 12 ambulances were involved in the response.
One helicopter with paramedics and St John's Medical Director Dr Tony Smith spent a short time on the island assessing the scene.
Lane said staging points were set up at Whakatane Airport and Whakatane Coastguard base to assess and triage patients. Patients had a injuries ranging from critical through to minor.
"It has been an exceptionally complex scene and our paramedics, both frontline and working in our Clinical Communications Centre, have done an outstanding job in very difficult circumstances."
Prime Minister Jacinda Ardern and the Minister of Civil Defence are travelling to Whakatane tonight.
"I know that there will be a huge amount of anxiety for those who had loved ones on the island at the time," she said.
Ardern would not provide any details about who the dead person was.
"We don't have full clarity at this point and you'll understand why we're loath to get into speculation."
The eruption happened at 2.11pm when about 50 people were visiting. Some were tourists from the Ovation of the Seas cruise ship which was visiting Tauranga.
The captain of the ship has told passengers that a group of guests and one crew member was on the island.
He told passengers on board that they're waiting on information from the Government to see what's happened, a passenger said.
The cruise ship will not leave the port until the missing passengers return.
Seven rescue helicopters were scrambled to the island after the eruption, including one which was tasked with flying blood from Auckland to Whakatane Hospital.
As news spread of the eruption, emotional families of those affected gathered at Whakatane wharf. One woman wrapped in an emergency blanket doubled over and wept while another looked out to the island and cried.
People covered in ash and lying on stretchers could be seen being dropped off by helicopters at Whakatane Airfield to be taken to hospital
As boats covered in ash returned, an NZME reporter at the scene said the wharf was becoming chaotic.
Videos are also emerging from people who were on or near the island showing people crowding to get off and onto boats. A damaged and ash-covered helicopter near the crater could also be seen sitting near the edge of 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.
I don't know if everyone survived
A tourist who was at the edge of the crater 30 minutes before it erupted was in his boat and about to leave when it exploded.
"[The] boat ride home tending to people our boat rescued was indescribable," Michael Schade tweeted.
He posted a video to Twitter of the aftermath and people clambering to get off the island, and said he was praying for them.
He said his mother helped a woman who appeared to be in a critical condition.
Another tourist who had just left Whakaari posted a video of the aftermath of the explosion to Instagram.
She can be heard saying: "The craziest thing in our life. We went to the volcano for an hour. Ten minutes after we left on the boat, it erupted.
"The boat turned around to try to help people. I don't know if everyone survived. We will see what will happen.
"I just hope no one is hurt. Thank God we are here and we are ok but still near the volcano trying to help."
Skydiving over the eruption
Skydive Tauranga directors Guillaume Calmelet and Tristan Webb were performing tandem skydives during the White Island eruption.
Each of the directors was jumping from a Cessna aircraft in tandem with crew members of a visiting cruise ship.
Calmelet said the sky above was clear as they made the journey to jump at 12,000 feet but the view was drastically different when they were falling through the air.
"This big white cloud came out of nowhere. We've never seen anything like that."
For the past roughly 18 months he had been working there he always wondered if maybe White Island would erupt, he said.
"I actually never thought it would come so quickly."
Webb said as he leapt from the aircraft he could see the "plume developing quite rapidly" above the island.
Under the parachute the view was clearer still as they made a steady but fast descent down, he said.
During free-fall one of the highlights of the jump was looking out to the view of White Island, he said, as usually people could make it out by the sight of smoke emitting from it.
"I always look for it when we exit the aircraft," he said.
"By the time I was under the parachute … It was huge"
Webb said he had never seen anything like it before.
"It was absolutely incredible," he said.
"I watched it the whole way under parachute."
Stay out of the ash fall
Civil Defence is telling people to stay out of the designated restricted zones, where ash fall may be a problem, and is warning people who live close to the island to stay indoors.
GNS Science said the eruption created an ash plume around 12,000 feet above the vent and volcanic ash had covered the main crater floor.
"Ash fall appears to be confined to the island, although there may be a small amount reaching East Cape in the coming hours," GNS Science duty volcanologist Geoff Kilgour said.
"Our monitoring equipment is still operating, and while we have seen a steady decline in activity, there remains significant uncertainty.
"Currently there are no signs of further eruptions, but we will continue to closely monitor Whakaari/White Island for further signs of activity."
The Volcanic Alert Level is at 3 – indicating a minor local eruption.
The biggest cruise ship to visit Tauranga
Ovation of the Seas passenger Nigel Walker and his family from Wollagong, south Sydney said his thoughts were those who were injured and unaccounted for. "It's tragic...I was only saying to my mother-in-law before the cruise that White Island would be a great pĺace to visit. "
There are 4579 passengers and 1595 crew on board the vessel.
The ship is on a 12-day voyage from Australia and had called at the Bay of Islands and Auckland after leaving Sydney last Wednesday.
It was scheduled to arrive at Wellington on Wednesday before cruising around the South Island and then back to Sydney.
'The worst eruption I've seen'
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.
