The largest-ever study into New Zealand's lakes has found 45% are in poor health or worse.
The situation is particularly dire in the North Island, with 80% of lakes in bad health.
The Lakes380 project, led by the Cawthron Institute and GNS Science, has been a five-year project looking at the health and history of lakes across the country.
Researchers physically sampled 10% of the country's lakes and used that information to build a picture of the rest.
The study by the Cawthron Institute and GNS Science looked at the state of our precious water basins. (Source: 1News)
They found 27% of lakes were considered poor, which meant they were murky and had high levels of nutrients and algae.
A further 15% of lakes were considered very poor, with swimming not recommended.
Meanwhile, 2% are exceedingly poor, with extremely high nutrient and algae levels.
Cawthron Institute freshwater scientist Susie Wood told 1News that the results showed a "really sad" situation.
"It's a huge number of our lakes that are now no longer safe to swim in or harvest kai from," Wood said. "You know that's 1700 lakes, so we've got a huge task ahead of us in terms of restoring our lakes."
The research project has involved 70 researchers from Aotearoa and around the world. It's significant because, previously, fewer than 5% of lakes were regularly monitored.
The worst regions in the study were Taranaki, Gisborne and Hawke's Bay, where 90% of lakes are in poor health or worse. The best regions are Southland, West Coast and Tasman.
Wood said there was a huge gap in knowledge for many lakes.
"A huge amount of lakes that we've forgotten about or have no scientific information on."
She said they worked closely with iwi to sample and learn about the lakes.
"Working with iwi has been absolutely the most rewarding part of the project. They're incredibly connected and knowledgeable about their lakes," the scientist said.
"It was very enriching for us to learn from them but also to help them understand some of the science around their lakes."
Margie Ward-Holmes Little of Ngāti Tama said it was great to be part of the project.
"It's a reality check, really isn't it, with our environment. We need to be aware of, perhaps more so, of what we as adults are doing with the land and to preserve it better."
Researchers didn't just look at the current health of the lakes. They also took samples of the soil under the lakes to learn about their history.
Those core samples are now housed at GNS Science in Wellington, where they've been analysed and archived.
Speaking to 1News, GNS Science lead scientist Marcus Vandergoes said there were about 2km of core samples at the facility.
"They usually go back just over a thousand years of time," he said. "All the layers in these core, they all represent a different event. It's like pages in a storybook."
The core samples could show, for example, what fish or plants used to live in the lake.
Vandergoes said it was important for researchers to figure out why some lakes are in such a poor state.
"Understanding how they have changed through time to get to that state and get to that condition is incredibly important."
Wood said there are a number of factors behind the deterioration of lakes.
"The main reasons that lake quality is deteriorating is due to changes on the land around our lakes. So what happens on the land also influences what goes on in the lake".
"And then also the introduction of non-native species... and also climate change now is a big one."
This is the first of a two-part story on the Lakes380 project. Watch 1News at 6pm on Monday for what can be done to restore the country's lakes.
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