Wellington Scoop

Green slime from multiple sewage leaks

sewage 1

by Kerry Wood
For six years in the 1990s I was the Wellington City Council’s Drainage Maintenance Engineer. More maintenance was clearly needed, but the manager’s objectives were effectively limited to minimising costs and obscuring problems.

Some years after I resigned, my successor told me that my survey records had been destroyed. This has since been confirmed by public data: no information on manholes which I had surveyed.

Another check I used to make was on places needing regular maintenance, to be sure the job was being done properly. I recently restarted checking one of them, but soon concluded that a stream entering the stormwater system is rarely or never cleaned. This means that stormwater drains will be steadily filling with gravel, and removing this will be far more costly than doing the maintenance.

I have long been aware of multiple local sewage leaks. A prominent indicator of leaking sewage is a green organic slime forming on roads and other hard and fairly horizontal surfaces. It often dries in sunshine, and I sometimes still see it on local streets. More recently, I have become aware of growing sewage problems in Lambton Harbour, which make public safety risks very likely.

The worst green slime example I have ever seen is on the north side of the Whairepo Lagoon (top photo).

sewage 2

There are more examples on the south side. The presence of green slime on this scale raises serious questions.

Such indications are also commonplace in the public-access areas of Lambton Harbour from the Railway Station to Te Papa, Oriental Bay and probably into Evans Bay.

marina slime

At the Marina next to the Freyberg Pool, there are private warning signs of slippery surfaces but with no public mention of health risks from the same cause: green slime.

The source of sewage in the Whairepo Lagoon will be an old stormwater pipe nearby: both sewer and stormwater pipes become more leak-prone with age. Both work at low pressures, and when both are damaged in multiple places they share their duties very effectively.

In Wellington, stormwater is seen as needing overflows to manage flooding, which means that either sewer or stormwater drains can contaminate the harbour. I have no experience of other systems and cannot be sure whether this is reasonable, but I am confident that it is unreasonable on the scale used in Wellington.

Another source of sewage in the harbour was reported on 29 September in a Dominion Post article on water quality, with the heading ‘Two-thirds of NZ river sites can make you sick.’ In the Wellington region, 51 stream and river sites were monitored. The Kaiwharawhara stream at Ngaio Gorge was in the worst national percentile for faecal bacteria and nitrates. The stream has a footpath beside it, through Trelissick Park. Below the path is a couple of kilometres of fairly large sewer, its condition clearly very poor – its location is given in the DomPost article ‘at Ngaio Gorge.’

small pipe

A smaller pipe in the same area is a bridge across the stream, connected to a pipe lying at footpath level. It is steel, in appalling condition, and was clearly in use when it was ‘repaired’ with a small white plastic bandage. Another bridge, a little further upstream, suggests that it may now be disused, but if this is correct, why has the old pipe not been replaced?

Two other connections to the Kaiwharawhara sewer are far more dangerous, but in a very different way. Two tunnels contain pipes used to drain sewage from Ngaio and Khandallah. I entered both in the 1990s, carefully, with an experienced advisor and a gas detector. Sewer gas, notably hydrogen sulphide, is very dangerous, and both tunnels had locked gates.


Today, one tunnel has no lock at all.

gate unlocked

The second tunnel has a lock but is not locked, with green slime beside a concrete structure, probably containing a pipe in poor condition. The bent bars suggest that people have been risking their lives in the tunnel.

Wellington’s ‘main interceptor’ sewer system runs from Johnsonville to the Moa Point treatment plant, with two parallel sewers south of the inner city. Some 70 pumping stations lift water to this system, and sewage in the system flows in large pipes on a very gentle gradient. This is necessary to reach Moa Point without more pumping. In Wellington, the arrangement needs tunnels, making access difficult, and safe access much more difficult. One section in a tunnel is from the Hospital to Kilbirnie, with very restricted access. Other sections are relatively safe, such as twin culverts from Kilbirnie to Moa Point, with frequent manhole access.

Almost 12 months ago, at the junction of Willis and Dixon Streets, a section of the main interceptor system collapsed, spilling millions of litres of sewage into the harbour. Repairs were very slow, and the reason will have been safety. Something similar happened in 2019, reported by Scoop on December 22. Nothing obvious has been done to address multiple, large-scale failures in the main interceptor system, and an apparent wait-and-see policy ensures future collapses. As the Wellington Water chief executive put it, ‘It’s a very tricky situation because the collapse is a long way down underground.’

Another main interceptor system problem is at Sar Street, part of the Johnsonville system. Sar Street joins the Hutt Road at the junction with Thorndon Quay, then turns right, heading up to Wadestown.


At the turn is a steep bank, on a WCC drainage reserve leading up to a main interceptor access point. Sewage has been flowing down the bank at intervals, probably in heavy rain, and has cut a ditch in the bank. The ditch has recently been partially filled with concrete, with a new manhole at the bottom, its purpose unclear. It cannot be effectively diverting sewage into the local system because ditch-cutting has clearly occurred recently. A reasonable assumption is that most or all of the ditch-cutting sewage is still entering the stormwater system and draining into Lambton Harbour.

Some City Councils object to the government’s ‘three waters’ proposals, but Wellington city councillors can see the advantages: the sooner the better.

In the meantime, public safety measures in Wellington should not be ignored: warning signs where needed in Lambton Harbour, and urgent security improvements in two tunnels.


  1. George, 13. December 2021, 12:36

    The issues raised in this article are clearly cause for concern. WCC needs to prioritise this work over other nice to have projects and projects which carry less public risk.

  2. Dave B, 13. December 2021, 20:37

    I hope WCC and Wellington Water take action on these defects which Kerry has identified. I confess I have walked through one of the two Trelissick Park tunnels many times (the one that permits access from both ends), and have never smelt even a whiff of hydrogen sulphide inside (it is hard to miss as it assaults the nose like rotten eggs). However I am aware that the nose can become de-sensitized to Hydrogen Sulphide after prolonged exposure, which is when things can get dangerous. I hate to say it, but perhaps there should be a better lock on the gate to keep urban explorers like me out.

  3. TrevorH, 14. December 2021, 7:24

    I don’t think Wellington’s problems will be fixed by a more remote and unaccountable management structure such as that envisaged under the Three Waters scheme, which more than sixty councils oppose. Wellingtonians have to address the failure of their local governance at the ballot box next year.

  4. Kerry Wood, 14. December 2021, 7:50

    Dave. You are ahead of me: I didn’t realise, or had forgotten, that one of the tunnels is open at both ends. But yes, locking out people like us is still a good idea. I wonder if it needs another padlock?

  5. paul, 14. December 2021, 12:46

    As a year round ocean swimmer who checks the LAWA ‘can I swim here’ site daily, I can confirm that more days than not our harbour is not fit for swimming. The merest hint of rain and it’s ‘code brown’

  6. Steve Doole, 23. December 2021, 9:20

    My brother spent years with the Wellington City Council, including time on sewer and drain replacement, probably before the time Kerry was involved. He mentioned jobs not being done properly and gave it up. He changed to the Porirua City council, which was more hassle to travel to, but increased job satisfaction.