A large yellow buoy worth a six-figure sum is still missing from the Kāpiti Marine Reserve despite calls to fishermen to keep an eye out for it.
The water quality monitoring device used by the Wellington Regional Council stopped transmitting data last Sunday.
It is programmed to "phone home" if it drifts from its mooring or suffers damage, but so far no alerts have been received.
Council spokesperson Stephen Heath said it's likely the buoy has sunk to the bottom of the reserve.
"It's a valuable piece of equipment worth several hundred thousand dollars so we are of course keen to retrieve it and get it back on duty."
A joint council, Niwa and Department of Conservation operation is underway to scan the seabed in an effort to recover the equipment.
The buoy was installed on the border of the Kāpiti Marine Reserve last November so it could monitor the impact of land activity on marine life - supporting conservation programmes with easily accessible data.
According to the council's website, Greater Wellington senior coastal scientist, Dr Megan Melidonis said: "It measures sediment levels which increase during heavy rainfall due to erosion intensified by urban development, overgrazing, and riparian clearing. This sediment enters estuaries and coastal environments and smothers marine life."
The buoy would act as a key component to the Greater Wellington Coastal Monitoring Programme which focuses on the causes of environmental impacts and identifies the success of ongoing restorative projects, the council said.