Tunnel hides dirty secret at Brown's Bay beach

Geoff Mason, general manager water services for North Shore City Council, shows off a model of the tunnel. Picture / Paul Estcourt

The grassy strip beside Browns Bay's beach hides a secret weapon in the fight against pollution of the North Shore waterfront.

Underneath it, a kilometre of tunnel - big enough to walk through - has been bored by machine over the past 18 months.

The purpose of the $12.5 million tunnel is to temporarily store the equivalent of two Olympic-size swimming pools of sewage which in heavy rain would have otherwise burst out of sewer manholes and pumping stations and flowed down the streets to the beach.

At yesterday's official opening of the project, North Shore City Council infrastructure and environment chairman Tony Barker said the tunnel had already saved two pollution spills in recent days. The rain on Anzac Day had half-filled the tunnel.

"It is preventing wet weather overflows on to the beach and we hope to cut the average number of them from 12 to at most two a year."

Mayor George Wood said the tunnel was part of Project Care, which had resulted from residents complaining about beaches being closed because of pollution.

Residents were pleased with the network upgrade, despite an increase in rates to fund the 20-year works programme, he said. Building storage tanks and tunnels eased the peak loads on the network by storing increased sewage flows, especially after cloudbursts when rain can leak into pipes and greatly increase the amount of sewage.

In October, work starts on a $5 million tunnel between Browns Bay and Torbay, which aims to double the capacity for future growth in the northern East Coast Bays