Mr Jamieson only had eyes for the youngsters on the cliff, where they struggled to figure out what to do - whether to duck into a cave behind them or jump into the water.
"I'm thinking, 'These are the kids of my best friends. One of them is my wife. F***'."
He made a split decision, shouting to the youngsters to jump into the sea.
"I'm just trying to figure out - do I get them back in the cave, or is the cave going to collapse? Or do I get them in the water? So I'm saying, 'I want you back to me! Just get in the water!'
"At least if the rock's falling, the water's going to protect them."
One of the youngsters decided to run back down the cliff, while the others - including William - dived into the sea.
It was then that Mr Jamieson saw a piece of rock "as big as a microwave" narrowly miss William.
William, a pupil at Sumner School, described the moment the rocks started "swaying".
"I just saw all the rocks come down. My mates were yelling, 'Get out, get out!' Then dad was yelling, 'Get in the water!'
"I jumped into the water and went under and I just thought, 'God, that was a dumb idea.' All I've been told is I almost got killed by a rock the size of a microwave. I got away pretty lucky."
William, who wants to be a police officer one day, said although it was big experience, he was not feeling scared.
"I feel pretty all right because most of Christchurch is pretty used to it now."