Passengers on a harbour cruise were given a rare treat when dolphin calves were sighted in Akaroa Harbour this week.

Hector's dolphins in Akaroa Harbour.

The Hector's dolphin calves were spotted swimming with their mothers between Bush Bay and the salmon farm, half way across the bay in Akaroa Harbour.

Hector's dolphins are classified as endangered by the World Conservation Union and seeing them is rare, which Black Cat Cruises Sales and Marketing Manager Natasha Lombart says is exciting.

"Because of their coastal habitat and slow reproductive rate they are particularly vulnerable to entanglement in fishing gear, especially gill nets, so we never fail to get enthusiastic when calves are spotted," says Ms Lombart.

Black Cat Cruises skipper Julian Yates says seeing the calves and their mothers was a "real treat".

"That there were two babies' with two mums was just fantastic."

The female Hector's dolphins have one calf every two to three years. The calves are 50-60 centimetres long at birth and stay close to their mothers who provide them with milk and protection for about a year until they are old enough to fend for themselves.

Hector's dolphins were spotted near the entrance to Akaroa Harbour playing with a female Orca and her calves last week.