A 60-metre slip is threatening the Whangārei Whau Valley Dam’s emergency spillway.

The hillside at the top of the slip is falling away, with a crack that’s 3m deep and half a metre wide at its top, to the northeast of the dam accessway and recreational car parking area. The crack is about 30m long. A further secondary “relaxation zone” above the slip is also moving.

Whangārei District Council (WDC) infrastructure committee chairman Simon Reid said the situation at the dam was “quite a concern”.

The dam and surrounds have been shut to the public since March 28 due to public access safety concerns.

WDC water services manager Andrew Venmore said the slip had been kicked off with Cyclone Gabrielle’s huge rains and then ongoing wet weather which meant groundwater levels were at a record high.

The one-hectare slip was heading down the hillside towards the spillway at its base.

Venmore said there would be significant consequences for the dam if the whole slip failed catastrophically and came down blocking the emergency spillway, but there was a very low chance of this. It would have to happen in conjunction with a major one in 500 year-plus weather event, which would potentially create problems with the amount of water in the dam building but having no emergency spillway to overflow.

He said the threatened emergency spillway was designed to cope with a one in 10,000-year weather event and had never been used since the dam was built in 1969.

The emergency or auxiliary spillway is Whau Valley Dam’s overflow backup. The dam’s primary overflow is its giant plughole-like bellmouth spillway, designed to cope with a one in 500-year weather event and in the dam at the western end of its front wall.

Venmore said that over time the slip’s soil would dry and stop moving, but it was currently very wet and still moving several centimetres a week.

“It’s slowed down a bit but is still moving,” he said.

Venmore said the slip did not mean the whole dam was about to fail with people downstream at risk as a result.

The dam, more than six decades old, is Whangārei’s main drinking water source, storing 1.8 million cubic metres of water, weighing 1.8 million tonnes.

Venmore updated councillors on the slip at a briefing meeting on Thursday.

WDC has put 14m-deep boreholes into the slip to monitor action, using a helicopter to lift the rig into place.

There have been delays getting contractors on site to do the bore drilling, due to major demands on their time from around the country due to Cyclone Gabrielle damage.

Data is being collected from the boreholes and the slip is monitored weekly.

Venmore said he was aware some visitors were frustrated about not being able to access the dam and its surrounds for recreation.

He said the council was working as quickly as possible to remedy the situation.

Forestry on the slipping hill was harvested by the council about five years ago.

Reid said the hill’s current pampas grass vegetation covering sent warning signals.

“Pampas grass doesn’t let water get away,” Reid said.

Venmore said the hill’s profile was lower than it was originally due to a lot of its soil being used to build the earth dam.

Meanwhile, a second 2009 slip in the dam’s southwestern end is causing issues, its sediment bringing extra water quality processing issues for the council.

Venmore said dam monitoring included regular drone flyovers.

■ Local Democracy Reporting is Public Interest Journalism funded through NZ On Air