The South Taranaki Bight has New Zealand's only known blue whale foraging ground, Dr Leigh Torres says.

In 2015, the first year of a three-year survey, Dr Torres' group saw 33 blue whales. This year they saw 68 different individuals, in multiple sightings over nine days.

They saw five mother and calf pairs, and two male whales having a race - possibly to show off and attract mates.

The whales looked thinner than they should be, and Dr Torres doesn't know why. Many wore scars from predators or collisions with vessels.

They will undoubtedly be affected if iron-sand mining goes ahead in the bight, Dr Torres told the Environmental Protection Authority hearing on seabed mining this week.

For whales, hearing is more important than sight. Sound from the iron-sand mining operation proposed by Trans-Tasman Resources could disrupt their actions or drive them away.

They feed on the marine crustaceans called krill, at depths of 70m-200m. Mining could increase the amount of fine sediment in the water and affect the presence of krill.

Mining would also increase the number of vessels in the bight. Vessel strike is another danger for the marine mammals.

Dr Torres' researchers have placed five hydrophones in the sea area between Farewell Spit and Cape Egmont. They record whale calls and help trace whale movements.

Recordings from January to July show a fairly constant whale presence, and Dr Torres expects to find the same for the rest of the year.