ARRIVED: With his feet in the Nettlebed system and his head in Stormy Pot, Kieran McKay was among a team of cavers who have linked Mt Arthur’s massive cave system.

The country's deepest cave has been found near Nelson on Mt Arthur with discovery of a link between the range's Stormy Pot and Nettlebed systems.

The about-10 kilometre passage drops 1200m, making it the deepest cave in the southern hemisphere.

However success has a wry twist - the link between the two systems was a 10m shaft used by caving teams as a urinal.

The mystery link between the two systems was found by cavers who have repeatedly returned to the mountain over the past three years. Members of the same team discovered the upper Stormy Pot system's entrance on Mt Arthur's western flank in early 2011.

The latest expedition was launched on January 23 when a group of seven cavers flew into the area. Two days later, and after three members of the team left for other commitments, the remaining cavers broke through the joining passage.

Caver Kieran McKay said the two systems were only five metres apart.

The discovery was aided by the team's sponsored operation with Red Bull Media last October which saw them drop dye into the Stormy Pot stream and light a kerosene fire in the "Rover's Return" camping area, at the then-known base of Stormy Pot.

Mr McKay said the stream disappeared into a sump, but smoke from the fire blew into a mound of rocks and could be smelt in the lower Nettlebed system.

Cavers spent two days digging aside fallen fault-line rubble to discover the link which Mr McKay proved to be a shaft the exploring team has used as a urinal during previous underground stays.

The discovery was great for New Zealand caving, he said.

"It's a real boost. There is a lot of exciting work going on underground right now - it's really the golden age of caving in New Zealand," he said.

The cavers had surveyed the system and planned to make a through-trip of it in a fortnight. They could not negotiate the link during the team's recent exploration as they could not get up the connecting shaft, he said.

The attraction was charting unknown territory and solving some of the country's big underground mysteries, he said.

Mr McKay said they were indebted to the New Zealand Speleology Society, the New Zealand Geographical Society, SPARC and other sponsors - and Nelson businessman Andrew McKenzie for producing "the best lightweight food we have ever eaten".

"We may have finished this project but it started in the 1960s and has involved hundreds of people and underground hours."