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Aucklander Tommy Cook arrived in Herald Island shortly after 7pm after hearing the explosion.
Herald Island is 25km from central Auckland, and near Hobsonville, reached by a causeway.
Cook said the fire on the boat had been extinguished since he arrived at the scene.
The person, who he believed had managed to escape the boat, had been taken to hospital.
"One person who lives on the boat had moored it just off the island.
"Someone who was on the shore said it looked like it was beginning to drift, when the explosion happened."
There had been talk that an LPG tank on board the boat had caused the explosion.
Cook said several St John Ambulance vehicles remained at the scene, along with two fire trucks and several police cars.
One local he has spoken to, said they felt the explosion as well as hearing it, despite being in a house several streets back from the waterfront.
A Greenhithe resident told the Herald they didn't see any smoke or fire from the scene of the explosion.
"We heard and felt the explosion, I thought it was like a cannon going off and it was very loud.
"From the bottom of our garden we were able to hear people calling for help and shouting, and see the scene from one of the nearby wharfs.
"Our neighbors also heard and felt the bang, and they are a bit further away from the location than us."