The whale spotted by fishermen off the coast of Whanganui on Tuesday was a humpback, the Department of Conservation has confirmed.

Philip Jolly and Mike Silcock were fishing in a 7m boat about 5km off the Whanganui River mouth when they spotted the whale just over 50 metres away and heading north. It stayed near them for about 20 minutes.

Les Judd from DoC said biodiversity ranger Sara Treadgold had gone out with Wanganui Coastguard to try and find the whale, but had been unsuccessful.

"They were keen to take photos of the whale to compare them to our database of whale sightings," Ms Judd said. However, DoC was able to confirm from Mr Jolly and Mr Silcock's photos that the whale was a humpback (Megaptera novaeangliae), approximately 14m long.

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"Humpbacks grow up to 15m long, so this was definitely an adult."

Ms Judd said humpbacks were fairly common around New Zealand at this time of year. They begin heading north after spending the summer feeding in Antarctic waters.

She said DoC would like to hear of any whale sightings off the Whanganui coast, including any photos taken.

"Whale movements are not that easy to monitor; and there are some whale species that are still a bit of a mystery."

For humpbacks, photos of their tail flukes are ideal. Each whale has a different black and white pattern on its tail fluke, so individuals can be identified.

However, Ms Judd urged people to be cautious about getting too close to whales to photograph them.

"We'd love people to report whales to us, but would hate for them to harm or injure any whales."

-Whale sightings can be reported to 0800 362 468.