A group of University of Otago students discovered an entangled humpback whale while out on the ocean conducting field work yesterday.

The whale, sighted about 10km east of Karitane off the Otago coast, was found with rope wrapped around its body and tail and was trailing a buoy which appeared to be from a fishing vessel.

The group reported the encounter to the Department of Conservation (DoC), which wanted to hear about any other sightings as soon as possible.

"At this time of year, humpback whales begin their northward migration from summer feeding grounds in the Antarctic to winter breeding grounds in the tropics, sometimes taking them past the New Zealand coast," said Dr Will Rayment, of the University of Otago's Department of Marine Science.

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"With species like humpbacks and southern right whales now recovering after the ban on commercial whaling we are seeing more and more whales in our coastal waters."

But that also meant that problems like this could become more frequent.

"More whales means greater overlap with human impacts such as fishing, and an increased risk of entanglements like this one," Rayment said.

"We need to start thinking about where and when these overlaps will occur so that we can manage the conflicts before they become too serious.

"It's pretty distressing to see a large whale entangled like this, knowing that it is suffering and that the consequences might be very bad.

"It was spending more time at, or just below, the surface than I would expect, suggesting that its diving ability was inhibited."

Anyone who saw the entangled whale should call DoC's 24-hour hotline 0800 36 24 68.

DoC advised that no one should attempt to cut free the whale themselves as it was very dangerous.

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DoC leads teams trained in disentangling whales using specialised equipment and these trained personnel would attempt to disentangle the whale.

The entangled humpback can be seen off the South Island's east coast in the coming days as it makes the annual humpback whale migration north to tropical waters.