Freshwater researchers are warning that the North Island's rivers and lakes are under attack.
Hornwort may sound like something from Harry Potter but the invasive species is wreaking havoc on our waterways.
"Lake Rototoa has the best example we have of submerged native plants in the region," said Auckland Council senior freshwater biosecurity advisor Belinda Studholme.
But in its depths, an infestation of the country's worst invasive submerged plant species has begun.
"It's depressing yeah," Studholme added. "It's classed as the worst suppressed weed in freshwater... you can just have a piece the size of your fingernail and that will start a whole new viable population."
Hornwort spreads like wildfire in lakes like this, forcing native species into a battle for territory they eventually lose.
"They just push the native plants deeper into the water and same with mussels - and then they reach a point where light can't penetrate down. So the native species die. And then the mussels keep being pushed further down until there is not enough oxygen to sustain them," Auckland Council senior ecologist Ebi Hussain said.
The water quality of Lake Rototoa is among Auckland's finest. And while the hornwort spread is considered mild here, it's already covered seven of the lake's 110 hectares. And Auckland Council say it's caught it early.
"There is hope, we've got a real opportunity here - where in other lakes, it just gets away on you," Studholme said.
"So we're lucky we've got this opportunity - and we're getting there to hopefully make a difference."
A spray boat has been driven up from Rotorua - and looks like something straight out of the Florida Everglades.
It's dropping the herbicide Diqua to stop the spread before it goes too far.
"We're lucky the concentration required to kill hornwort doesn't kill the native plants in the lake and it doesn't affect fish or the kākahi (freshwater mussel)," Studholme said.
But while there's hope for Lake Rototoa, Department of Conservation living water science lead Katie Collins said hornwort's grip on Te Ika-a-Māui / North Island's lakes is strong.
"It is a significant threat," she told Newshub.
"It does dominate lakes and rivers - can be found in the Waikato River, in Lake Taupo, it's very widespread."
And to get rid of it completely?
"It would take a coordinated effort from all the agencies to achieve that," Collins added.
Auckland Council, at least, is making a start.
