The water level in a leaking dam near Kaitaia is being lowered so engineers can take a closer look.
However, the Far North District Council says there is "almost no chance" the earth dam will collapse.
The 20m-high Kauri Dam, which straddles Kauri Creek on the edge of Herekino Forest south of Kaitaia, has been leaking for some time - "donkey's years", according to some - but a routine inspection in mid-February found the leak had changed.
An independent dam safety engineer was notified, who requested the water level be lowered by 1.5m to reduce pressure and allow him to assess the leak and make recommendations on what to do next.
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Far North Mayor John Carter said the leak was in the face of the dam just above the "toe", an extra area of earth reinforcing the dam's base.
Lowering the water level started on Monday and was expected to take seven to 10 days. The extracted water is being fed into Kauri Creek via a manifold to avoid scouring out the creek bed.
Mr Carter said council contractors and staff had been monitoring the leak for some time and keeping the dam safety engineer updated.
"He says there is very little to almost no chance of a sudden failure of the dam ... We're just being cautious," Mr Carter said.
The dam is owned by the Far North District Council and was built in 1964 as a backup water supply. It is about 20m high, 125 long at the crest, and 40m-50m thick at the base. It is accessed from Okahu Rd.
The water in the dam is rarely used because of toxic algal blooms which occur in summer. It could not be used, for example, during the last major drought of 2009-10 when the Awanui River, the source of Kaitaia's town water supply, almost ran dry.