Heavy rain can see caves become like a "stormwater pipe" where people can be easily "swept away", a caving expert says.
It comes after a Whangārei Boys' High School student was swept away during an outdoor education and pursuits class that was hit by bad weather during a trip to Abbey Caves yesterday morning.
The class was made up of 15 Year 11 students and two adults, with 16 making it to safety.
The boy's body was recovered last night.
Caver Peter Crossley told 1News in summer, the caves are safe enough for children to go through them.
He said as you enter deeper in to the caves, the water level rises from ankle-deep to neck-high.
"In summer, that’s alright. It’s warm water and you can take most of the family through there as long as they’re adventurous. In winter, it will be different."
But in heavy rain, people can be swept away at a moment's notice.
"If it’s a downpour outside, it’s mainly farmland outside so in saturated conditions, the water will quickly run off the hillsides, into the cave at the top and water can come up within moments, almost roar down the passage.
"It's like in a sewer pipe, a stormwater pipe in the city. There’s nothing really to hold onto on the sides – you’ll just get swept away."
Crossley said the size of the group had also made the situation more challenging.
"If you had a party of 15 and only two strong people, experienced people, it would be very difficult for the ones at the front to know what the ones at the back of the 'conga line' were doing."
Crossley said while regulations are in place with "all the safety checks and the boxes to tick and all the rest of it", the guides need to "have more authority over stopping".
"The guides have to be able to say, when they get to the entrance of the cave, 'No, it's not safe today, we're gonna go back.'"
Despite yesterday's tragic accident, Crossley said it's "fun" if you enter "the right caves and you know what the weather’s like".
"I think it's good to have these kids going into caves. There's all sorts of organisations that do it, and very safely too. But yesterday, not after seeing the weather forecast, no.
"I wouldn’t go in myself."
He advised people going caving to go with an expert, and to bring a torch.
"A cell phone torch is not good enough… You don’t see the caves and it’s not safe."
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