The threat of the Waiho (Waiau) River wiping out the Franz Josef sewage ponds is back on the radar after the 95% of the river's main channel diverted into the neighbouring Tatare Stream.
The Westland District Council's sewage ponds, which sit on a historic river plain, are sited where the Waiho River is now swinging north, away from its normal bed.
On Thursday, the West Coast Regional Council said the ponds were not at immediate risk, but the situation is serious and could change for the worst.
The river moved after heavy downpours on the West Coast last weekend.
Another 200mm rain event is anticipated at Franz Josef this weekend, with the area under a heavy rain warning from Friday night to Saturday morning.
In April 2016, a "moderate" rain event caused flooding after the Waiho River broke through a stop bank on the northern edge of Franz Josef.
It wiped out the district council's wastewater treatment plant and also the Scenic Group's Franz Josef Mueller Wing hotel complex, which remains abandoned.
The regional and district council's insurers settled an insurance claim for an undisclosed amount with Scenic's insurers in early 2023, following a $30 million claim as a result of the 2016 event.
After the flood, the district council stuck with the existing wastewater treatment plant and ponds site, although it explored other options for a new site at the time.
Westland District's chief executive Simon Bastion said on Thrusday that the latest development with the river cutting into the north bank just past the ponds was worrying.
"We're concerned the breach to the Tatare continues to increase. The biggest issue is the waste water treatment plant and the ponds," Bastion said.
At this stage, the council did not have a detailed strategy to shift sites.
He said previous options that were explored after 2016 were hamstrung by the difficulties council had in the wake of the fraud of council's former district assets group manager Vivek Goel, which affected several infrastructure projects.
Goel was jailed after being found guilty of corruption and bribery in March 2023.
Bastion said council would be meeting immediately with West Coast Regional Council officials to discuss the implications and management of the existing site.
As things stood there was no immediate plan to shift, although council had it in its long term plan.
"The 10-year plan highlights that the ponds need to be replaced at some time."
Bastion said they would "revisit" a solution previously proposed for the site, which meant they wouldn't be starting from scratch.
"We will be kick-starting a programme to find a good location."
Bastion said the limited time left on the existing site's 15 year consent was a factor, with about seven years before the consent runs out.
The issue now was the long lead-in time including gaining the necessary consents and starting construction — at least a three-year time frame.
"Time is the critical factor at the moment".
Meanwhile, both councils jointly briefed Emergency Management Minister Mark Mitchell last weekend about the natural hazards threatening Franz Josef.
Mitchell arrived in the midst of the State of Emergency on January 19-20.
Regional council chief executive Darryl Lew said it was a timely opportunity with councils and the community to formally approach the Government about the longterm future of the area, following the proposal to abandon the south bank stop banks outlined in the Waiho River Management Strategy released in October.
By Brendon McMahon, Local Democracy Reporter
LDR is local body journalism co-funded by RNZ and NZ On Air
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