Bay of Plenty lifeguards on 'high alert' after Tonga volcano eruption and Cyclone Cody

"Do not go into the water at all."

That's the message from Tauranga lifeguards who say swimming at the beach for the next couple of days will still be too dangerous.

A massive undersea volcanic eruption in Tonga and Cyclone Cody were causing strong and unpredictable surges along the Bay of Plenty coastline, with local lifeguards on "high alert".

Despite the warnings, people have been ignoring the safety messages, with some needing help from lifeguards to get out of "powerful" thigh-deep water.

Civil Defence New Zealand issued a national advisory of tsunami activity yesterday after the eruption sent ash, steam and gas up to 20km into the air and has caused untold damage in the Pacific nation.

The eruption could be heard by people throughout the Bay of Plenty and beyond.

Papamoa Surf Life Saving Club president Andrew Hitchfield said the message was clear: "Don't swim".

Surf Lifesaving New Zealand's Allan Mundy warns people the sea will be too dangerous to enter after the Tongan eruption and Cyclone Cody. Photo / NZME

"Stay out of the water for the next day or two until things calm down," he said.

"Do not go into the water at all. It is just too dangerous."

Hitchfield said lifeguards were on "high alert" at least for the next 24 hours.

He said it had been a busy two weeks and advised people to know their limits in the water.

"It has been a pretty horrific season for drownings to date. We hope we can stop more tragedies from happening."

Mount Maunganui Lifeguard Service chairman Jamie Troughton said: "We seem to have missed the main event and dodged a bullet. But there are still some big waves."

At the Mount's main beach yesterday , waves reached 4m and closed signs were put up.

Troughton said the swell was expected to drop "pretty quickly" but "there is still going to be a lot of water moving".

"The message is to have extra respect for the water."

Surf Life Saving NZ search and rescue manager Allan Mundy was also warning people not to walk their dogs along the beach during high tide in the evenings.

"All our lifeguards are off duty at that time," he said.

"We have water rushing right up to the sand dunes. Walk along the top of the beach, don't venture down."

Mundy said he assisted some people from the water yesterday. "Even at thigh-deep, they [waves] have got some power".

He said he was disappointed with the "handful" of people still swimming despite the many warnings.

Surf lifesaving competitions have been cancelled all around the Bay of Plenty due to the water conditions, he said.

At Mount Maunganui beach near Tay St a young family were seen swimming despite the warnings. The father declined to comment.

Tay St beach. Photo / Zoe Hunter

Civil Defence emergency management Bay of Plenty director Clinton Naude said the national advisory to stay out of the water, off the beaches, shore areas, rivers and harbours had been extended until further notice.

"There is still volcanic activity in Tonga and our thoughts are with everyone there."

Naude said the advice was for people to move out of the water to reduce their risk of harm.

"However, there has not been an imminent life safety risk on land, no evacuations required, and no risk of flooding on land or coastal inundation, therefore Emergency Mobile Alerts have not been necessary."

Emergency Mobile Alerts were used for tsunami warnings where there was considered to be a significant life-safety risk, he said.

"We are keeping a close eye on the situation and working with the National Emergency Management Agency and GNS to ensure we are giving the community the latest information."

Naude said precautions are also still in place for Cyclone Cody and he advised people to check the MetService website and Facebook page for the latest weather updates.

MetService meteorologist Ashlee Parkes said models initially tracked Cyclone Cody towards the Bay but it was now looking like the cyclone would pass east of the country.

"The Bay of Plenty will see little to no weather associated with the system."

Parkes said the main thing was keeping a close eye on sea and swell conditions in the region.

Waves to the northeast were expected to reach up to 5m, she said.

"Combined wave conditions will be hazardous...

"There is potential for significant sea surges, rip currents, and coastal inundation.

"We are encouraging people to stay out of the waters because of the large swells with the system as it passes through."

But she said the sea conditions will deteriorate late Monday.

Parkes said tomorrow was looking to be "quite warm", with Tauranga recording a high of 31 degrees.

A panoramic view of Mount Main Beach. Photo / Zoe Hunter

However, she said it will be a "bit windy", with strong southwesterlies across the country.

The Mauao base track was closed yesterday as a precaution due to "strong and unpredictable surges", the Tauranga City Council said in a media statement.

Kaysi Fredericks, who is an administrator for the BOP & Waikato Weather and Natural Events Facebook page, said she had about 3000 new members overnight on Saturday due to the eruption.

New members from Rotorua, Hamilton, Murupara and as far as Christchurch, joined the social media page after seeing updates being posted of the Tongan eruption.

"I had no idea it was going to be shared nationally."

Fredericks said many people heard the "sonic boom" on Saturday when the eruption occurred in Tonga.

"They were commenting that their whole house was shaking, their windows and everything."

Tauranga Bridge Marina manager Tony Arnold put out a warning earlier in the week for everyone to tie their boats up for the weekend in case Cyclone Cody hit.

"We are pretty protected from anything coming from the east. The swells from the volcano eruption didn't come this far down.

"We have got a bit of change of tidal direction but nothing damaging."

Maketū Coastguard president Shane Beech said there was visible surging on the beach.

"We've got some really massive sea."