He said the crash happened at the southern end of the lake, where there were "rocky outcrops".
"The force of the impact flung the woman into the boat's cabin, and the man was ejected through the windscreen and into the water.
"It was extremely lucky he wasn't knocked unconscious when he was thrown into the water. He must have been working on pure adrenaline, because he was still calling for help as we were getting him out of the water. Both have sustained severe injuries and have a long road of recovery ahead of them."
Mr Grouby said he could not determine how the crash happened as "a number of situations" could have caused the incident.
"They were on the lake in the pitch black, so there are all sorts of different situations and factors that could have led them to crashing into the cliff face.
"For example, if the pair were navigating by GPS, that would have had a part to play because, while GPS is good for telling you where you are going, it doesn't tell you what's right in front of you.
"In pitch black, they wouldn't have been able to rely on their sight."
He said it was vital people were prepared before going out on the water.
"Things like making sure they have a Coastguard membership, completing a day skipper course and letting other people know when they're out on the water can all save someone's life."
In addition to the Coastguard crew, two ambulances and the Rotoma Fire Brigade attended the incident.
Mr Grouby said volunteers from Rotorua Lakes Coastguard did an outstanding job recovering the people from the boat in "trying circumstances".
