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The group, headed by Ross woman Biddy Manera, has been moving rocks by hand to channel some of the flow from Jones Creek away from the artificial lake and stop it flooding the memorial gardens.
Flooding has already inundated and killed dozens of saplings, and left the pagoda standing in water.
Water levels in the lake fell after the volunteers arranged rocks to channel more of the flow through a culvert, but rose again after the latest demolition job by an alleged vandal.
Westland Mayor Bruce Smith says the ongoing row is causing tension in the town, and while the district council is prepared to help resolve it, the regional council is the arbiter when it comes to waterways.
West Coast Regional Council compliance manager Heather McKay said Ms Manera's group was not breaching any rules or consents by diverting part of the flow of Jones Creek, because the work was considered minor.
But the position of whoever had been wrecking their rockwork needed clarifying, and council compliance officers would be making a site visit to investigate, Ms McKay said.
Ms Manera said the volunteers had called the police 111 line more than once when a man had appeared at the scene while they were working and "thrown rocks around".
Police had yet to respond, she said.
The Ross Lake is an old mine pit which was filled with water with regional council consent, after the mine closed in the 2000s.
The Chinese community supported local efforts to beautify the area with trees and a pavilion in memory of the miners who worked in the area in pioneering days.
But a slip in Jones Creek last year caused the lake to flood, damaging the planting and flooding the pavilion.
By Lois Williams
Local democracy reporter