Six student divers were treated to a surprise in Otago Harbour yesterday.

A southern right whale about 25m in length came within reach of the group, around midday near Wellers Rock.

Dive Otago open water scuba instructor Gina Watt was showing where the dive would begin when she saw the whale's massive tail lift out of the water.

The mammal swam under the divers' boat and hung around for about half an hour.

Onlookers ashore also enjoyed the experience, capturing footage of the whale from different vantage points.