The weather bombs that hit Auckland in March are estimated to have poured more material into the city's main water dams than the slips caused by the Kaikoura earthquake.

But unlike the slips that fanned out along the Kaikoura coastline, the power-packed storms poured huge volumes of sediment into four dams in the Hunua Ranges, leading to Aucklanders being asked to save 20 litres of water a day.

Documents released to the Herald under the Official Information Act suggest much of the sediment came from land slips at a large pine forest in the Hunua Ranges.
Watercare boss Raveen Jaduram today said native forest slips also played a significant part.

"Early estimates are that a greater volume of material slipped into the water storage reservoirs than the slips at Kaikoura which blocked the state highway and Main Trunk Lines," says one document. It did not give volumes for either event.

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The documents show the greatest volume of sediment poured into Cosseys, Wairoa and Mangatawhiri reservoirs where there was a mix of native and pine forest. Less sediment poured in the Mangatangi reservoir, which only has native forest in its catchment

Aerial spraying of a herbicide on a forestry block closed the Wairoa dam in June last year, and two dams were closed for four months in 2014 after traces of a herbicide got into the dams' supply lakes.

Despite the herbicide and siltation incidents - and a report recommending an end to commercial forestry in the Hunua Ranges - Watercare is continuing with the harvesting of pine trees.

Documents show that in February this year, Watercare received a report commissioned before the storms on managing the threat to water quality from commercial forestry operations.

The report, from consultants Margules Groome and Ahika, said the harvesting of pine trees midway through their growing cycle would decimate native undergrowth and put the water resource at risk of sediment and herbicide contamination.

The consultants recommended an end to harvesting pine trees and using the trees as a "nurse crop" to shade out exotic weeds and help the regeneration of native trees and plants. Over time the pines would decay.

"Abandoning harvest of the radiata pine in the Hunua Forest...will almost eliminate the risk of contaminating the water with sediment," the consultants said.

In January this year, Watercare purchased the private 1800ha commercial pine forest in the Hunua Ranges to control the harvesting programme and better protect the source of two-thirds of the city's water supply from herbicides and siltation.

Jaduram said Watercare planned to prioritise native regeneration, starting with buffer zones between the reservoirs and commercial plantings and slopes with a high risk of erosion. The first stage involved planting 230,000 natives trees over the next two years. Millions more trees will be planted later.

He said Watercare had on-sold the forestry rights for about 740ha in lower risk locations for harvesting over the next 25 to 30 years to a commercial forestry company. About 80ha has recently been harvested.

Abandoning harvest of the radiata pine in the Hunua Forest...will almost eliminate the risk of contaminating the water with sediment

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Council chief operating officer Dean Kimpton supported Watercare's approach to regeneration, saying as custodians of water quality in Auckland it was best qualified to decide to harvest trees in certain areas.

The document also show that following the storms an SOS went out from Watercare to council to urgently do whatever is possible to stabilise the slips and get some cover established before winter.

The council and its water company has responded by using helicopters to spray a mixture of polymer, moss and grass seed onto exposed areas to immediately reduce silt run-off by 85 to 90 per cent and produce grass cover within 12 months.