Special liners will bring a new lease of life to Metrowater's wastewater pipes in Otahuhu and Mt Wellington.

"The liners are like giant socks with open ends," says Metrowater Operations Manager Raveen Jaduram. "They are made of a felt containing resin, and can prolong a pipe's life by 50 years."

Metrowater contractors are installing 850 metres of the sock-like lining into a wastewater pipe in the Carbine Road, Mt Wellington area. Mr Jaduram says that in the past chemicals in the wastewater have eroded the bottom of the pipe, so the liner will make it more durable.

A longer section of pipe in Otahuhu will also be lined because it is cracked. The pipe runs for 1.3 kilometres from Chelsea Ave to Jane Cowie Ave, and even goes under the playing fields at Kings College. Mr Jaduram says contractors will carry out the work using a special method that means fewer disturbances to property.

"Instead of digging up large areas and disturbing roadways and private properties, the liner will be fed through the pipes using hot water. The water pressure inflates the liner and the heat makes the resin harden, like concrete. A special robot then goes into the pipe and cuts openings in the lining where other pipes connect to it. It's very efficient and low impact compared to alternative methods," he says.

Together the projects are costing $1.38 million. The Mt Wellington project will be finished by late July and the Otahuhu work by the end of August.

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