The aquifer, 20 metres below the seafloor between Timaru and Ashburton, is one of the shallowest in the world, extends for up to 60km from the coastline and may hold up to 2000 cubic kilometres of water - about half the volume of groundwater across Canterbury.
Most of the water in the aquifer is freshwater that became trapped offshore during the last three Ice Ages, when the sea level was more than 100 metres lower than today, NIWA said, but it was also being replenished by groundwater flow from the coastline.
Researchers first had indications of a fresh water source there in 2012, after an offshore scientific drilling project found a combined mixture of salt and freshwater 50km off the coast and 50m below the seafloor.
That led to a 2017 voyage on NIWA research vessel Tangaroa to carry out further investigation. An electrical source was towed behind the ship and a line of recievers behind that recorded different signals depending on the electrical resistivity of the ground, NIWA said.
It is one of the few times a significant offshore aquifer has been found in the world, NIWA marine geologist Joshu Mountjoy said.
"One of the most important aspects of this study is the improved understanding it offers to water management," Dr Mountjoy said.
He said not a lot was known about what happened to fresh water beyond the coastline.
"We don't know how much is out there and how much is being released into the ocean," he said.
"That's our buffer to drawing in saline water into the aquifers as we take water from near the coast, so the more we can understand about the offshore part of these, the more we can manage them."
Dr Mountjoy said it could become a new freshwater resource for the region, which is susceptible to drought.
The next step is to take samples for analysis.
"At the moment we have used remote techniques, modelling and geophysics. We really need to go out there and ground-truth our findings and we are investigating options for that."
Dr Mountjoy said there were several places around New Zealand facing significant problems with their groundwater such as Christchurch and Hawke's Bay, which were feeling the pressure of increasing populations and regular prolonged dry periods.
"Hawke's Bay is an example of a region needing to manage what they're dealing with onshore. They've only got half the picture if they don't know how far out it goes, and how much is leaking into the ocean.
"We need to set the groundwork in place for the future. 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."
The study is an outcome of the MARCAN project and was funded by the European Research Council, the NZ Ministry of Business, Innovation and Employment, US National Science Foundation and the German Research Foundation.
