A humpback whale caught up in what appears to be a craypot line has been spotted off the lower South Island.

Photo: Catherine Peters DOC

It was reported to the Department of Conservation by a fishing boat crew who saw the whale at Knife and Steel Harbour, between Big River and Waitutu River on Saturday.

The line appeared to have the wrapped around the whale's pectoral fin and tail stock and was trailing 20m to 30m behind it. The whale was moving and could travel up the west or east coast of the South Island.

DOC is appealing for sightings of the whale so its teams trained in disentangling whales can attempt to free it.

"People seeing the whale can assist our rescue response by staying with the whale, monitoring it and advising of its exact location for our disentanglement team to get to it," DOC ranger Mike Morrissey said.

But people are warned not to try to free the whale themselves, as it's dangerous, or to cut any line and floats, which would make it difficult for the team to carry its work.

Mr Morrisey, who leads the South Island large whale disentanglement team, asked boaties to not get close to the whale or do anything that would disturb or harass the whale.

In late March a humpback whale entangled in fishing line was reported off the Otago coast. The whale was then seen off Kaikōura several days later and the whale disentanglement successfully cut the rope from the whale.

Cutting whales free can take several hours and has to be done when sea conditions are not too rough

The procedure for disentangling whales, called kegging, involves using grapnel hooks to attach rope and floats to material entangling a whale to slow it down and tire it out.

When the whale is sufficiently exhausted, the team edge along the rope until they can reach over with a long pole and a range of various knife blades to cut away whatever has entangled the whale.

Humpback whales migrate north through New Zealand waters at this time of year to tropical breeding grounds.

Anyone who sees the whale should call DOC's 24-hour hotline 0800 DOCHOT / 0800 36 24 68.