Monarch Cruises guest ambassador AJ Carrick said they had about 10 people on board when they came across seven adult southern right whales near Harington Point.
"I don’t know exactly why they were there, but they seemed quite relaxed and they were kind of more like socialising, as opposed to feeding, because they were on their side a lot and waving their pectoral fins and you could hear them singing or chatting to each other as well, which was really cool.
She said everyone on board was very excited and busy trying to take photos of the rare occasion.
"They were all pretty frothing. I’ve heard of so many people hanging over the side to get a better look.
"Everyone I’ve talked to was like, ‘oh my gosh’. They were well stoked.
Miss Carrick said she had heard from others, that the whales were also spotted further up the harbour at Waipuna Bay, before turning around and heading back out to sea.
It was a special moment for all who were lucky enough to witness it, she said.
Documentary film-maker Bill Morris, of Port Chalmers, said he had been "lucky enough to spend quite a lot of time with southern right whales down in the Auckland Islands but yesterday was the first time he had seen one — let alone seven — in Otago Harbour.
"It was amazing just to hear the sounds, the reverberating sounds of their breathing, Mr Morris said.
"It gave me chills, to be honest, to hear those noises echoing around the harbour and to see those whales back in after such a long absence.
"It was, yeah, quite an emotional thing to see and I’m sure I wasn’t the only one who felt that today.
Department of Conservation marine technical adviser Hannah Hendriks said it was "an exciting sighting because it was unusual to see a group of this size, this close to mainland New Zealand.
"Usually, we only see individuals or mother and calf pairs.
"Their population suffered dramatic losses from whaling, and they came close to extinction.
"Hopefully this sighting is a good sign the population is slowly recovering, and that we continue to see more southern right whales around our coast.
She said during this time of year, they were typically found at their breeding grounds around the subantarctic Auckland and Campbell Islands, but there were occasional sightings around mainland New Zealand.
"It’s exciting to see whales like this, but we remind people to please share our coasts with whales responsibly by giving them plenty of space and following the Marine Mammal Protection Regulations.
