In January, six Australian divers followed the river back to its source in Kahurangi National Park where it emerges out of a mountain, beneath a cliff.
They swam into the hole and down the submerged cave, with diver Craig Challen making it to a depth of 194m, without reaching the end.
His descent is the deepest cold-water dive on record.
The expedition took five tonnes of equipment.
One diver at a time swam, in pitch blackness, against the current of the 6-degree water.
They placed four "habitats" along the passage where they could sit in trapped bubbles of air, eat and drink and wait out the decompression needed because of the extreme depths.
Challen said they took down containers usually used for bulk liquids, upturned them and filled them with air.
For the nine-hour dive he was actually only swimming for 25 minutes - the rest of the time was spent sitting in the habitats.
The cold water required electrically-heated dive suits.
Challen said it was not known how much deeper the cave went but the team was coming back next January to find out.
The Pearse Resurgence, as the cave is known, was first explored in the 1970s.
Diver Dave Weaver died exploring the cave in 1995.
Challen said it was his second expedition with leader Richard Harris.
Asked why he would willing swim hundreds of metres down pitch black caves, Challen said: "We have a saying 'If you have to ask the question you don't understand the answer'."