Whales in Auckland's Hauraki Gulf are consuming 3 million microplastics every day.
A study led by the University of Auckland discovered that on average, whales ingests about 25,000 microplastics per mouthful.
The tiny plastics, less than 5mm in length, have been found in every marine environment studied to date, but how many animals are exposed to each day has been hard to estimate.
"The numbers are big and… make us really stop and think about what's in the water," Associate Professor Rochelle Constantine said.
The research found that the majority of the microplastics being ingested by whales in Auckland's backyard are in their food, such as zooplankton.
Constantine told 1News only about 1 in 1000 come from the water they swallow.
"Everything within the Gulf is compromised by these microplastics 'cause it's pretty close to the beginning of the food chain consuming them in the first place.
"Anything that eats plankton, or eats things that eat plankton, will be ingesting microplastics."
She said it also shows we shouldn't be measuring plastic pollution in our marine environments through water samples alone.
"Most of the microplastics are in the zooplankton, so we need to start using them as a measure of contamination."
Microplastics come from everything, including cigarette butts, clothes, plastic bottles and even rope and buoys used in the water as they become worn down.
Co-chair of the Hauraki Gulf Forum, Nicola MacDonald, who's also mana whenua, was shocked by the findings and said we need to pay careful attention to the "groundbreaking study".
"We desperately need to get on top of plastics pollution as it's infecting entire whole ecosystems that we have in our marine habitats,” she said.
"I know all of us - both New Zealanders and mana whenua - have a deep love for our seas and oceans; what we don't love is when we see the risk to this taonga species through our own endeavours.
"It's human impact we need to get on top of," MacDonald said.
"We need to look at infrastructure and really have some strong monitoring around how we dispose of plastics."
Constantine said: "We are just getting more and more [microplastics] over time, so I guess that goes back to the beginning - stop making plastics and if you do make plastics, we have to work out a way that we contain them.".
“They’re not going to go away but we can do something now that maybe in 100, 1000 years’ time, that this is not such a big problem.”
Researchers don't know what the health impacts of the microplastics are on whales, but Constantine said studies are being done overseas.
"It's not a zero consequence," she said.
"They live in the backyard of our biggest city and that comes with some cost, and the ingestion of microplastics is probably quite a big cost for them."