"It was very unusual, I've never encountered anything like this before," Maketu fire brigade Chief Fire Officer Shane Beech said.
The Rotorua rider, in his early 20s, was treated for possible concussion and a back injury, before being taken by Te Puke St John Ambulance to Tauranga Hospital for observation.
Mr Beech said the jetski was being launched for an afternoon's fishing about 1pm when the explosion took place at the boat ramp, a few hundred metres from the river mouth.
"He turned on the key and 'boom', it exploded."
It was understood the men had just topped up the jetski's fuel tank at a service station on their way to the boat ramp.
"Petrol got into the hull and all the fumes gathered inside the jetski," he said.
When the brigade arrived, the man was conscious on the ground next to the boat ramp and being treated by St John paramedics.
Mr Beech said the explosion blew the fire out and they were left to make sure that there was no more flammable material.
Bystanders told him that they heard the sound of the jetski being fired up and then the boom of an explosion. He said the man was lucky there were people able to reach him so quickly.
The explosion happened in about 1.5 metres of water.
"He was in the confines of the launch area and people were able to come to his assistance.
"He lived to tell the tale."
