It was among the most prolific types of litter found during a survey of 20 sites around the region in 2022.
In 2019, 0.76 litres of plastic litter was found per 1000sq m. But in 2022, the amount ballooned to 7.59 litres per 1000sq m.
Nationally, the 2022 audit found there had been a 335.9% increase in plastic litter — from 7.35 litres in 2019, to 32.04 litres in 2022.
In Otago, industrial sites were associated with the highest number of litter items and weights, while car park sites recorded the second-highest.
Retail sites were associated with moderate-to-high numbers of litter items and weights, and residential sites were associated with high litter volumes.
Public recreational spaces were associated with low litter volumes.
While cigarette butts and vaping material was the most frequently identified material type per 1000sq m, it accounted for the smallest litter volumes in the region.
Plastic was the second-most frequently identified material type and was associated with high litter volumes.
Keep New Zealand Beautiful chief executive officer Heather Saunderson said the results were "surprising and alarming", considering they came off the back of years of Covid restrictions.
Developed in consultation with Stats NZ, the Department of Conservation and the Ministry for the Environment, the audit involved experienced environmental field researchers collecting and analysing litter in specific fixed sites and recording the key metrics of the litter collected, as well as brand data.
"Despite the efforts of tidy Kiwis nationwide, the audit results really speak to the fiction of New Zealand's clean green image and illustrate the need for government, industry and Kiwis to take immediate action,
"The 2019 audit provided a baseline for litter intelligence in New Zealand, painting a picture of how litter impacts our communities," Ms Saunderson said.
"The 2022 results serve to highlight that the need for educational and behaviour change programmes, when it comes to litter in New Zealand, is more important and urgent than ever."
The 2022 audit was an extensive piece of research that was critical for New Zealand, Ms Saunderson said.
"The importance of the audit for both land and ocean litter management are immense.
"The findings will help inform national and local policy development, guide industry strategies and enable Keep New Zealand Beautiful to identify the originators and activities that generate land and marine litter, and to guide our efforts accordingly.
"We’re focused on education, behaviour change and working with leaders of industry who are committed to sustainability."
