Natasha Lynch and her husband Matt Hall were enjoying the last day of the long weekend boating off Whitianga and taking photographs when they heard an "almighty crack".

A section of the archway at the popular Coromandel tourist spot of Cathedral Cove had just collapsed sending rubble crashing to the ground about 15m below about 2pm on Monday.

The debris, which was the size of two cars, sent out a "massive soundwave" and only narrowly missed a group of beachgoers.

"My husband's face was absolutely horrified, it was like a cannon booming as the sound carried out," said Ms Lynch. "It would have missed one of the tourists by about a metre but one woman ran back into the cave and stood over the rubble and was looking up at where it had fell from and I was just yelling at her to get the hell out of there."

Ms Lynch said a pile of rubble lay inside the archway about 3m wide and 1m high.

She was critical of local police, who showed a "lack of enthusiasm for the whole thing" when she called on a mobile phone for help and tried to warn many of the non-English speaking tourists away.

"I'm really worried that's just the beginning and the whole thing could eventually collapse. There could have been a multiple fatality there."

"I thought they would send a boat around to ferry people around because it is too dangerous to swim around but people were just running through trying to get back to the stairs."

Whitianga police could not be contacted for comment yesterday.

The Department of Conservation's Hauraki area manager, John Gaukrodger, said geologists were inspecting the site which had seen problems with erosion at cliff faces and side walls around the cove in the past.

"But this to my knowledge is one of the first instances where it has come directly from above from the sort of apex of the arch itself."

Mr Gaukrodger said access to parts of the cove was now restricted.