Protecting Our Water Supply

12 July 2011

The main water supply lines into Wellington are being upgraded so we can better cope and recover after a big earthquake.

Using PVCO pipes to replace an old cast-iron water main in Kelburn Parade

Using PVCO pipes to replace an old cast-iron water main in Kelburn Parade

The water in the 77 reservoirs around the city is supplied in bulk from Greater Wellington Regional Council. The treated water that eventually comes out of the taps of Wellington's homes and businesses is pumped from treatment plants in Upper Hutt, Wainuiomata and Lower Hutt. We also have a back-up supply at Petone.

The supply lines from each of these treatment plants cross the Wellington faultline at least twice.

We have been planning and working in the last 15 years to replace old cast iron and fibrous cement pipes with more resilient materials such as PVC, polyethylene, ductile iron or steel.

Some 35 percent of our underground water network now comprises earthquake resilient pipes. Our wastewater system is up to 16 percent.

The City Council's Infrastructure Planning Manager, Maria Archer, says the pipe upgrades are in conjunction with extensive work to quake-proof the reservoirs.

"At the moment 90 percent of the city's water is stored in reservoirs that are protected by autoclosing valves and about half are built to the latest seismic standards.

"Our long-term plan is to have as much water stored in the city as possible.

"We want to have reservoir water available to local residents within a few days of a big quake.

"Because there is likely to be a significant gap in our ability to supply water immediately following an earthquake, I advise everyone to follow civil defence guidelines and prepare yourself with at least three day's drinking water supply per person.

"Water pipelines and faultlines are not an ideal combination but fault lines are very hard to avoid in Wellington which is why, over the past 15 years, we've been looking very closely at ways to get around this problem and increase the resilience of our water supply."

Greater Wellington's Utilities and General Services Manager Murray Kennedy says a number of projects have already been completed to strengthen the bulk water supply to Wellington city and restore water more quickly.

Among the projects:

  • The pipeline from Ngauranga Reservoir to Aro Valley and Mount Cook has been diverted and connected to reservoirs that serve the central, southern and eastern parts of the city, so it avoids two faultline crossings.
  • A valve has been installed in the Wainuiomata to Wellington pipeline before it crosses the Wellington fault at Thorndon. This will allow the pipeline to be shut-off for repair at Thorndon, but still supply water to Ngaio and Onslow reservoirs.
  • The pipeline to Karori's main reservoir has been buried beneath the bed of the Kaiwharawhara stream, making the pipe less vulnerable to flooding if the lower Karori dam breaks in an earthquake.
  • A new emergency water supply point has been installed in Khandallah, where tankers and residents will be able to get water in an emergency.

Mr Kennedy says Greater Wellington is continuing to work closely with the City Council to identify where more improvements can be made to the bulk water supply into Wellington.

"We're looking particularly closely at locations in the city with the potential to provide water in an emergency."