The tunnelling phase of Watercare’s Central Interceptor project is complete. 

With one final turn of the 5.47-metre-diameter cutterhead belonging to the Hiwa-i-te-Rangi Tunnel Boring Machine (TBM), the tunnelling was done. Four years after beginning its journey from a site next to Māngere Wastewater Treatment Plant, Watercare’s Central Interceptor project was another step closer to completion as the TBM broke into a shaft 16.2 kilometres away at Point Erin Reserve, Herne Bay.

The new infrastructure will see an 80 per cent reduction of wet weather overflows into streams in central Auckland. Water quality at beaches will also improve as flows from the old combined stormwater/wastewater network are carried away by the new Central Interceptor tunnel to the treatment plant for safe processing.

Auckland Mayor Wayne Brown, plus 200 project staff and contractors, watched on a large screen as New Zealand’s largest wastewater tunnel was completed. Opera singers sang Puccini’s ‘Nessun dorma’. Special effects smoke and flames added to the spectacle. There were hugs, and a few tears of relief and joy.

Once the official photographs and media interviews were over, the cutterhead was cleaned so crews could disassemble the TBM and remove it from the 30-metre-deep shaft. It took three weeks to take away all 18 gantries that make up the almost 200-metre-long TBM before it was trucked away to a storage yard in East Tamaki to await the next assignment.

Tunnel cleaning and stripping is now underway. Around 100 staff are removing ventilation systems, high-voltage cables, locomotive tracks and other services so the tunnel can be water blasted. The last traces of tunnel spoil must be removed so thermal welding of the yellow plastic tunnel segment joints can take place. This will ensure the tunnel is airtight. It’s a long, careful process. Six rings make up a single ring. That’s more than 100,000 linear metres for the northern section of the tunnel alone.

Watercare’s Acting Chief Programme Delivery Officer, David White, says the successful completion of tunnelling is particularly impressive given the major difficulties the project faced as a result of the COVID-19 pandemic. 

‘It’s hard to look back and remember how challenging those early days were,’ says White. ‘We went into a national level four lockdown just days after we had launched our main TBM. There was constant rapid antigen testing and social distancing – all this when the tunnellers were trying to establish a rhythm and get to know each other.

‘We had to get special permission from the government to keep tunnelling, to prevent the TBM from getting stuck underground. And, in the background, there was huge financial pressure and a need to stick to the programme,’ adds White.

‘This was followed by supply difficulties; and closed borders meant we experienced a shortage of skilled, experienced staff.’

 It was the first time Ghella, the Italian arm of the Ghella Abergeldie Joint Venture, had worked in New Zealand and this is the longest single drive in the company’s 130-year history. Central Interceptor Project Director Francesco Saibene says despite the challenges brought on by the global pandemic, Ghella is delighted with the overall success of the project.

This, coupled with its recent attendance at the government’s Infrastructure Investment Summit, has given the company reason to stay in New Zealand. 

‘We are very pleased with the way the tunnelling operation has gone,’ says Saibene. ‘It was great to be part of the summit and to see firsthand how much work is happening behind the scenes. The government’s commitment to attracting investment should lead to more projects coming to market, and we’re keen to remain and get involved.’

The entire $1.6-billion Central Interceptor project is due to be completed in early 2026, but the benefits are already being seen.

In Māngere, Hiwa-i-te-Rangi’s launch shaft is now a massive pump station. Mayor Brown gave the order to start up the pumps on Valentine’s Day. Now, five German-made pumps each send combined wastewater/stormwater flows from the first half of the tunnel (from May Road, Mount Roskill South) to the nearby treatment plant at 1200 litres per second. A succession of heavy downpours has demonstrated that the new infrastructure is performing brilliantly.

Watercare Central Interceptor Commissioning Manager Axel Dumont says that in just four months of operation, around 93,000 cubic metres (equating to 37 Olympic-sized swimming pools) of combined wastewater/stormwater have been saved from spilling into the environment.

‘When we first switched the pump station on in January, we had nothing but blue skies for weeks. Typical! But then the first time it rained heavily, we were delighted to see that the tunnel and the pump station transported the flows perfectly and in manageable quantities for the treatment plant to handle,’ says Dumont.

‘Previously, the ageing Pump Station 23 at Hillsborough would overflow into the Manukau Harbour almost every time it rained. Likewise, Pump Station P25, Avondale, would discharge into the local streams in Blockhouse Bay. We’ve had seven significant rain events, and the tunnel and pump station has worked really well. A huge win.’

Elsewhere, there’s plenty of activity at the northern sites. At Keith Hay Park, Mount Roskill, hydraulic gates have passed commissioning tests inside a new plant room. This will allow transmission network operators, based across town in Newmarket, to control flows from the local wastewater system into the new Central Interceptor tunnel.

Shafts are still to be connected. Concrete cutters recently sliced through tunnel segments to connect the Walmsley Avenue shaft, in Sandringham, to the tunnel. All the work was done from inside of the tunnel.

Construction is still going on in Grey Lynn, Sandringham, Western Springs, Herne Bay and three sites in Mount Albert. The presence of basalt makes any excavation activity more arduous.

At Point Erin Reserve, work has started on the Herne Bay Collector, a new 1.5-kilometre tunnel that will pick up combined stormwater and wastewater flows from lower Herne Bay, and transport them to the Central Interceptor tunnel lying below the park and away to the treatment plant.

The $1.6-billion Central Interceptor is on track for completion in 2026.

‘It’s one of those rare beasts: a major infrastructure project that is being delivered safely, on time and almost to budget. If you look around the world, or within New Zealand, you’ll know how unusual and impressive that is,’ says White. ‘We’re not finished yet, but the end is in sight; we’re due to finish mid next year, and we couldn’t be prouder of the entire team.’

The learnings won’t be forgotten. Steps to share the knowledge within Watercare and the wider construction industry are already underway, starting with a fresh approach to contractor inductions.

In March, Watercare published its 10-year Business Plan to deliver for Auckland. It shows that as the Central Interceptor project successfully winds down, other major infrastructure projects will get underway, such as a replacement of the ageing Huia Water Treatment Plant and major upgrades to the Māngere Wastewater Treatment Plant.

With dozens of treatment plants, hundreds of pump stations and thousands of kilometres of pipes, Watercare must maintain assets from as far south as Waiuku to as far north as Wellsford. Over the next 10 years, there will be new work in every community.