Outdoor adventure instructor Kieran McKay and his five-member Extreme Cave Team have high hopes that the system will link up with the Nettlebed Cave.
The groups discovered the new Stormy Pot system after weather forced them to seek shelter on the western side of Mt Arthur.
They found a muddy entrance which led into a wider, higher system. It has so far taken them to a depth of 470 metres and extended about 2.5 kilometres.
Mr McKay said they had to abort the mission when the cave abruptly ended in a waterfall.
"We were in a long canyon, about 30m high, when we went around a corner and the water shot into blackness."
The stream appeared to exit the mountain about 800m further down its eastern slope, he said.
Mr McKay said the team carried "heaps' of rope and was eager to get back into the system. They would map it for future cavers.
They hoped it might lead to the Nettlebed Cave system at the head of the Pearse River. It has been explored to a depth of 890m and is 23 kilometres long. The Ellis Basin system is just over 1000m deep – New Zealand's deepest – and 33km long. "This one could even be deeper," Mr McKay said.
Source: Nelson Mail