Suddenly a group of up to 10 orca "appeared out of nowhere", Gehrig said. Footage of the encounter shows the curious orca nosing up to and diving under the boat, breaching and expelling air.
The five children on the boat were all under 10 and were "absolutely very excited", Gehrig said.
"But everybody was very excited, not just the kids. We're always looking and hoping but it doesn't always happen - we were extremely lucky this morning.
"We know it can happen but it's certainly not a regular occurrence. We might see them two or three times throughout the summer."
The crew believe there were two separate groups of the animals.
"You keep your distance but they come up - they're quite curious animals so they will come and have a look.
"Sometimes they're not so interested and will swim away but they weren't obviously fishing this morning."
The killer whales hung out in the harbour all day, and were sighted close to Akaroa township as late as 8pm tonight.
Guests had been swimming with the dolphins earlier but if they had encountered orca would have left the water, Gehrig said.
"It's not something you would want to risk. You're not allowed to swim with orcas anyway."