An aquifer has been discovered under the Pacific ocean off New Zealand’s coast, with scientists saying some of the South Island could draw on it for their main freshwater supply.

Joshu Mountjoy, a marine geologist at the National Institute of Water and Atmospheric Research (Niwa), said the discovery was one of the few times a significant offshore aquifer had been located anywhere in the world. As climate change begins to bite it could become a crucial source of freshwater for nearby drought-prone regions such as the Canterbury plains.

The aquifer is located 60km off the east coast of the South Island at a depth of just 20 metres – making it one of the shallowest in the world. Although the exact quantity of water it contains is not yet known, it could be as much as 2,000 cubic kilometres of water – equivalent to half the volume of groundwater across the entire Canterbury region.

The aquifer was discovered when scientific drilling in 2012 discovered brackish water – a mixture of fresh and sea water – leading scientists to investigate its source.

According to Mountjoy, the aquifer’s extensive resources were partly being replenished by groundwater flow from the coastline between Timaru and Ashburton.

However most of the fresh water became trapped offshore during the last three ice ages, when the sea level was more than 100 metres lower than today.

Mountjoy said there were several places around New Zealand facing significant issues with their groundwater, including Christchurch and Hawke’s Bay, where increasing populations and prolonged dry spells were putting pressure on supplies.

Last week a major drought was confirmed across the entire North Island, where in some places rain had not fallen for months.

“We need to set the groundwork in place for the future,” Mountjoy said. “Our primary goal is to help people manage their onshore resources. Our groundwater systems are a critical resource for society, they are increasingly under pressure, and we need every bit of information we can get.”