"We were having trouble with the water and one day a drop hit my wife on the head.
"The ceiling was bulging and we got a plumber in who opened it up and said 'God, I've never seen anything like this in all my life'."
The bees also produced a lot of honey, which O'Sullivan believes mice ate before it ran out and they decided to chew on the wiring instead.
If you see a swarm of bees, it's best you call an expert like Ibrahim Mohammad of Bee Swarm.
Swarms occur when a new queen is born and its mother leaves the hive, Mohammad says.
"They do move around, a swarm in the morning moves on before 11am. An afternoon swarm will hang around overnight and move on in the morning," he says.
"When they do move on, they normally end up in a roof cavity or people's walls and end up getting exterminated. It's important that we get hold of them so give us a call straight away."
Spring is an especially busy time for Bee Swarm as it's the time of year new queens are born. The business takes between 20 to 25 calls each day.
Mohammad removes swarms and re-hives them throughout Auckland for free but an extermination costs.
Swarms can be found in trees in a cluster the shape of a rugby ball or sometimes on the ground. Mohammad says swarms can be 10kg - around 20,000 bees.
If a swarm is on the ground, the beekeepers will lure the queen with wax frames and if they're on a branch, they gently shake them into beehive boxes.
O'Sullivan has been forced to remove part of the roof to remove the old hives and honeycomb.
He says it's a problem he could do without at 91 years old: "We didn't realise it was as bad as it was."
