Community groups are appealing a new scheme which aims to stop septic tanks leaking human waste into the Marlborough Sounds.
The Proposed Marlborough Environment Plan included a rule requiring Sounds homes with sewerage systems to either be part of a warrant of fitness scheme (WOF), or join a community sewerage scheme, within five years of its release. The plan was released in February.
Okiwi Bay Ratepayers Association, which challenged Marlborough District Council's new plan, said compliance came with a hefty price tag.
Queen Charlotte Sound Residents Association was listed as supporting the appeal, but a member was unable to confirm if the association was actively involved. Federated Farmers was also listed as supporting the appeal, but said it was not appealing the rule.
Okiwi Bay Ratepayers Association vice-chair Noel Curr feared residents would have to pay up to $5000 for a WOF test for individual septic tanks, or band together to pay $2 million for a communal system.
Residents believed if a WOF scheme went ahead, compliance officers might run soil tests, send in surveillance cameras, or uproot septic tanks to find out where systems were. Some of the bay's houses were 80 years old and had no known wastewater designs plans, he said.
"We believe that could cost between $2000 and $5000 a house."
The environment plan said systems under the new scheme would need to be inspected every five years. Okiwi Bay is the third-largest settlement in the Sounds, after Picton and Havelock, with 200 houses.
This meant residents could be forking out up to $2m a decade.
A consultant had told residents it would cost about $2m to put down a community system, which included $600,000 for a wastewater plant.
It is currently up to landowners to ensure their systems work.
Curr questioned why wastewater systems with a history of compliance could not continue as is. His own household system had been checked every six months for 20 years, and was "working fine".
"We're all for improving the water, but we want it to be cost effective."
An environment plan report writer said the council had received about 190 complaints on household sewerage systems in the last decade, suggesting some might not be functioning as well as thought.
There were also concerns that "poorly designed" septic tanks were discharging waste into the Sounds, following water quality testing.
A council spokesperson said the council could not provide details on the plan's new method, as a WOF scheme had not yet been developed, but there were a range of ways to address poorly performing systems.
This ranged from improved or more regular maintenance, or a system upgrade or replacement, which could come in a range of options.
Council environment policy manager Pere Hawes said the new approach would not be rolled out until the appeal was resolved.
"If the appeal is successful, the method would not be implemented."
Council environmental scientist Steffi Henkel said at a council meeting last month that Governors Bay and Robin Hood Bay would be investigated after returning high faecal results on days without rain.
Investigations were also planned for Ngakuta Bay, which had "significantly higher" faecal matter than other bays after rainfall.
Five bays in the Sounds monitored for faecal matter, or 56 per cent, were graded "fair", meaning swimming should be avoided during and after rainfall. Two sites were graded "good" and one was "very good".
Robin Hood Bay was labelled "poor".
Henkel said last week old septic tanks had been the source of faecal contamination in the past, and she suspected this had occurred again, but the council was waiting on genetic sourcing results to prove it.
The Proposed Marlborough Environment Plan, officially released on 20 February, brings three of the region's plans into a single document and defines what activities are appropriate in Marlborough's urban, rural and coastal environments.
Local Democracy Reporting is a public interest news service supported by RNZ, the News Publishers' Association and NZ On Air.