Boulders come crashing down Mt Maunganui's Mauao

Two booms, a huge puff of dust and then five hundred tonnes of dangerous rock came crashing down Mt Maunganui's Mauao today.

In a successful operation, a team of geo-technical engineers and an explosives expert sent hundreds of pieces of rock tumbling 232m to the mountain's base at midday.

One of the sizeable boulders struck a container that protected the toilet block of the camping ground but the rest of the debris landed in paddocks.

A crowd of about 200 gasped when the bone-rattling boom went off.

Seconds later there was a loud thud as one of the boulders bounced down the hillside, crashing into the side of a shipping container that had been put in the campsite.

Other boulders could be seen crashing through the undergrowth but they did not make it as far as the protective ring of containers.

Seconds after the explosion the crowd burst into a round of applause and a cheer went up.

Council site manager David Green said: "It went superbly -- we couldn't have asked for a better end."

The fractured rock, overlooking the camping ground and hot pools, was stuffed with up to 20kg of explosive charges.

The explosion was managed so smaller pieces of rock would cascade down the slopes and land harmlessly in the low-lying paddocks that were ringed with 47 shipping containers.

People took up grandstand views on the grass in front of Main Beach and along the Adams Ave cafe strip to watch the big blast. Adams Ave was closed off to through traffic all morning. Police and security guards were on hand to assist in crowd control.

The explosion was preceded by a succession of hooter blasts - one at 11.45am, two at 11.55am and then three at midday to warn people that the detonation was about to happen.

After the dust settled, abseilers descended down to the shattered site to scale off loose rock and check the impact. The base of Mauao remained sealed off for several hours.

Yesterday, most of the drilling was carried out on three main blocks of rhyolite rock but some smaller ones were also primed with explosives.

In a delicate operation, each hole was filled with a 200mm-long explosives stick which was packed into the rocks with dampened paper wadding to make sure the blast splits the rocks.

Today's big blast followed a rush against time to evacuate the populated area directly underneath the dangerous rocks.

The landslide threat was revealed after geotechnical engineers started monitoring the rockface after the bushfire in January.

Those checks revealed the critical front-facing rock had moved several millimetres over the past six months and was being squeezed out to the point where only half of it was left balancing on another rock.

- BAY OF PLENTY TIMES