Water testing samples are still being collected by helicopter at Lake Horowhenua at a cost to regional ratepayers of more than $14,000 so far.
Horizons Regional Council confirmed it had continued to use the method after threats were allegedly made against staff members at the lake edge last year.
The council cited health and safety policy as the reason the helicopter was being used and said work on the ground had been hampered by alleged "physical intervention, aggressive behaviour and threats" towards Lake Accord partners, usually Horizons staff and Lake Trustees, undertaking work or taking part in activities relating to the lake.
However, the New Zealand Taxpayers' Union said the helicopter costs were a waste of money that someone needed to be held accountable for.
"We were first tipped off on this wasteful spending about a month ago – we couldn't believe the council would waste money on a helicopter instead of waders," said the organisation's executive director Jordan Williams.
"We understand the council is trying to defend the spending because an individual has [allegedly] previously threatened staff.
"Who on earth had the idea to hire a helicopter instead of a security guard or the police?"
Horizons confirmed the helicopter has been used six times so far, costing from $2200 to $2600 each time.
From 2013 to early 2018, Horizons monitored Lake Horowhenua monthly by boat, which included the purchase of a vessel specifically for the sampling of the lake, due to concerns from locals around the potential transfer of weeds between lakes, chief executive Michael McCartney said.
"Monitoring by boat is cheaper in the case of monitoring Lake Horowhenua however, due to multiple [alleged] threats that have been made against staff that have been undertaking monitoring on the lake this form of monitoring is no longer viable from a health and safety perspective," he said.
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A Horizons document supplied to the Horowhenua Chronicle said aggressive behaviour to council staff during monitoring had allegedly occurred including verbal and physical threats and one council staff member receiving a "trespass notice". This had resulted in Horizons limiting some activity in and around Lake Horowhenua, the document said.
"Monitoring has largely ceased, although limited monitoring is being done by accessing the lake through the use of helicopters with permission from the Domain Board and Lake Trust. Science work has continued using information gathered through the significant amount of field work completed earlier this year and prior to that," the paper stated.
The council said halting of some activities was considered a temporary measure, but helicopter monitoring would continue until Horizons staff felt safe accessing the lake edge.
McCartney said it would be inappropriate to comment further because a case was before the courts.
Last August, the signing of a Lake Accord was celebrated. Horizons said that through this, considerable progress had been made in the restoration of Lake Horowhenua.
The Lake Accord is a collaboration led by the Lake Trust - a body that is elected to represent the beneficial owners.
Other partners include the Horowhenua Lake Domain Board, Horizons, Horowhenua District Council and the Department of Conservation.
A Horizons Lake Horowhenua Accord action plan document states that in 2013, five parties representing Muaūpoko iwi lake owners, community interests and statutory bodies agreed to work together "to provide leadership, halt the degradation and put in place remedial measures" on the lake.
In 2010 the lake was ranked as the seventh worst of 112 monitored lakes in New Zealand and is regularly closed for recreation purposes in summer months due to toxic cyanobacteria.
Between the early 1960s and late 1980s, coastal forestry clearance and swamp drainage, land use intensification, urban expansion along with effluent and sewage being discharged into it created serious degradation of Lake Horowhenua.
When the sewage was stopped in the late 1980s, water quality picked up for a while but began declining again a decade later.