Franz Hubmann, left, and Joe Nawalaniec upstream from a major slip (visible in the background) that has blocked the headwaters of the Tauherenikau River creating a new lake in the Tararua Ranges.

Three trampers believe they have discovered a new lake in the Tararua Ranges, which GNS Science says may have been caused by the November quake.

A massive slip into the seldom-visited upper headwaters of the Tauherenikau River, west of Masterton, has dammed the river, creating a lake up to 100 metres long and 18m wide.

Joe Nawalaniec, Franz Hubmann​ and Paul McCredie​ found the new water body during a trip up the river on Saturday and believe the valley's remoteness means it's likely they were the first people to set eyes on it.

GOOGLE EARTH The 'new lake' is at the red dot. The coordinates are: lat: -40.968234, long: 175.296307

The trio thought the slip probably come down after November's magnitude 7.8 earthquake which was followed by torrential rain the next day.

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"The slip was so big that we saw the top of it going several hundred metres up the side of the Beehive spur, from a kilometre downstream. It really was an impressive sight," said Nawalaniec, who is from Carterton.

JOE NAWALANIEC The slip is an estimated 300m long and may have come down following November's 7.8 magnitude earthquake and a heavy rainstorm the following day.

He and his two Wellington companions hiked for more than six hours to reach the spot.

"When we got there, the dam and fresh broken rock and trees rose metres above us, and [it] was a bit of a clamber to get up."

He said the condition of vegetation on the dam – with browned leaves still attached to the dead trees – suggested the rockfall had occurred several months ago, or around the time of the quake and the storm.

JOE NAWALANIEC Joe Nawalaniec's dog Floyd clambering over some of the massive boulders that came down in the slip. Floyd has claimed 'naming rights' to the lake, having reached the top of the dam first.

In the Kaikoura region, the devastating quake resulted in about 100 dams being formed by cliffs spilling into waterways.

But a GNS Science geologist said there had not been any previous reports of the quake creating dams in the North Island.

Engineering geologist Sally Dellow said it was possible the quake and storm had triggered the slip, but it might also have occurred for other reasons.

JOE NAWALANIEC The site of a major slip in the headwaters of the Tauherenikau River which has created a new lake in the Tararua Ranges.

"The lake behind it is not particularly big or deep – it hasn't impounded a large volume of water so there's no particular downstream hazard as a result of the slip," she said.

"If it was bigger we might talk to [the Department of Conservation] about the position of Cone Hut [several kilometres downstream] and whether that might be in danger but at this stage I'm not seeing that as being a particular issue given the description and the location."

Nawalaniec said he had never seen a slip that size – an estimated 300m long – in the 35 years he had been tramping in the Tararuas.

PAUL McCREDIE Franz Hubmann looks upstream from the dam face. The lake is estimated to be 100m long, 18m wide and 2-3m deep.

"As soon as I saw it I thought this is odd. Then I put two and two together regarding the earthquake and it reminded me of some of the [slips] that we'd seen down the Kaikoura coast – it was exactly that kind of slip."

Hubmann said the spectacular find was completely unexpected.

"We were expecting [the river] to straighten out and become easier going – and then this came along," he said.

JOE NAWALANIEC The trampers believe the dam face is stable, meaning the lake is likely to become a permanent water feature.

"It doesn't look it's going to give way suddenly. It looks like it's dammed up pretty permanently."

Nawalaniec said his dog, Floyd, who also made the trip, had claimed naming rights for the new water body because he was first to reach the top of the dam.

The dog had decided he wanted the lake to be named Floyd Pond, Nawalaniec said.