KEY POINTS:
The group on the Tauranga Coastguard boat was expecting a countdown, but instead they saw clouds of smoke then heard the bang of a controlled explosion on the slopes of Mt Maunganui's Mauao mountain.
A rockface the size of a small car and estimated to weigh 100 tonnes was blasted off the northern seaward side of the 232m peak yesterday.
The detonation was watched in the gloomy weather by a few boats and explosive and geotechnical experts.
Those on the Coastguard boat were told there would be a five-second countdown before the detonation.
Instead, the blast surprised everyone by going off just seconds after an announcement of "two minutes to go" from a Tauranga City Council staff member with a walkie-talkie.
Grey smoke filled the air around the top of Mauao as the sound of the bang reached the boat and chunks of rock cut a path down cliffs, across walking tracks and through vegetation.
At least three large rocks from two-thirds the way up the mountain fell to the water's edge.
The detonation was deemed a success by the council, which contracted geotechnical blasting experts Avalon to carry out the operation after a section of rock fell away from Mauao's upper slopes in November.
The rock moved as a result of natural compression, leaving a sizeable overhang which was discovered during a routine inspection and deemed a risk to walkers.
Geotechnical experts said if left untouched, it would come down on its own within five years.
Warren Aitken, the council's park rangers' team leader, said the risk to people walking on the tracks was high and there was no choice but to blast away the overhang.
More than 115,000 people used Mauao's tracks in December alone, and plenty of warnings, including security guards, were needed to keep people away from the mountain during the detonation.
"Just closing the Mount down to the public is a major exercise in itself," Mr Aitken said.
The rock detonation was the second carried out on Mauao in recent years. The first was required after fires in 2002 loosened large boulders, threatening the campground at its base.
Yesterday's detonation left one large rock embedded in the upper Oruahine track.
The council said last night that the rock would require further work to dislodge, probably by drilling and blasting, and the track would then need to be inspected and repaired.
It and the other tracks on Mauao were not expected to reopen before this afternoon.
