A pipe in Hobson Bay has been pumping sewage and water to treatment plants in Auckland for 98 years. Now, Watercare Services plans to keep part of it as a monument to the structure's vital role in the city's history.

One of Auckland's oldest eyesores - the concrete sewer pipeline across Hobson Bay - is to join the city's list of 14,000 historic sites.

Demolition of the 98-year-old main is expected to start in April, after commissioning of its replacement in the city's network.

Despite years of complaints about the chunky pipe on stilts being a blot on the landscape and a health hazard, Watercare Services plans to save a 20m section of it from the digger's jaws.

It will become a monument to the structure's vital role in the city's history.

The section is on a flat piece of land on the edge of the bay, near Ngapipi Rd.

"We had a number of calls from people asking whether we were going to leave it," said Mike Sheffield, Watercare Services' project manager on the $120 million pipeline replacement job.

"We felt this was a good opportunity to leave just one piece of it.

"It would not interfere with the other objective of clearing out the bay.

"It would be a place where you could see a marker board saying what it did and a picture of what it looked like in those days when it reached right across to the railway line there and on to Parnell.

"In the United States, you see a lot of historical markers and this is just marking the pipeline's presence since 1912 in Auckland."

The section of sewer to be retained is on public land, is below the road and is screened from view by big trees.

Grass will come to the top of one side of the pipe, which will be sealed.

In addition to the role for which it was built, the pitted and steel strapped top of the pipeline has, until recent years, served generations of bird watchers, walkers and joggers.

One of those was Auckland City Mayor John Banks. "I have wonderful memories of jogging across the pipeline in the days when it was safe.

"I'm supportive of keeping this piece of architectural heritage in place," said Mr Banks, speaking from the opening of the Dataline company in the 1908 vintage signal box at 66 The Strand, in Parnell.

Mr Banks was mayor early last decade when engineers said the pipeline had reached the end of its economic life.

Auckland debated options ranging from the cheapest - giving it a new concrete jacket for $26.5 million - to the dearer one of a tunnel, for $47 million.

Mr Banks said he was pleased the decision was made to go for the more expensive option.

A tunnel is being built 30m under the bay from Logan Tce, Parnell, passing under Ngapipi Rd and the Orakei Ridge to Orakei Domain.

* Carried waste to the Orakei holding tanks, now Kelly Tarlton's aquarium, where it was flushed out to sea.
*Later, helped to move about a quarter of the 304,000cu m of Auckland's daily wastewater to the Mangere treatment plant.
*A 20m section will be retained on public land, below the road and screened from view by big trees.
*Grass will come to the top of one side of the pipe, which will be sealed.
*The new $118.6 million tunnel is wide enough to drive a car through and will almost eliminate the more than 20 sewage overflows a year into the bay.