The remains of masses of landslides lie on the bed of Lake Tekapo, according to scientists at the National Institute for Water and Atmospheric Research (NIWA).
They now want to find out whether this means more landslides could occur and if so, whether they could trigger a tsunami.
If that happened, it could endanger local residents, tourists and a network of hydro electric power stations.
NIWA stressed there was no evidence of actual danger at this stage and it was doing mathematical modelling to see what sort of tsunamis could occur if there was another big landslide.
There have been instances of tsunamis triggered by falling rock in hydro lakes overseas, it said.
It noted the Tekapo area was prone to earthquakes.
Watch NIWA explain the research behind the Lake Tekapo landslides
NIWA marine geologist Joshu Mountjoy said the lake varied between 100 metres and 150 metres deep and had steep sides with thick layers of soft material, much of which had collapsed onto the lake floor.
"At this stage we don't have any evidence that there has been a tsunami in the past in Tekapo," Dr Mountjoy said.
"But now we can see how big the landslides have been in the past we can assess what would have happened previously and make some determination about what could happen in the future."
The landslides covered about half the 83kms of lake floor, he said.
Many had flowed about a kilometre and were tens of metres thick.
The research was done by scientists using state-of-the-art sonar equipment to collect high quality bathymetric maps.
Apart from looking at the lake bed, the research will help look 100 metres below the lake floor.