New port in Manukau Harbour ‘technically’ possible but many red flags

West coast weather and the complex and dynamic nature of the Manukau Harbour entrance make it a tricky proposition for a major new port site. Photo / Hayden Woodward

Making an access channel for shipping in the Manukau Harbour would take up to six years and cost up to $1.2 billion and keeping it dredged would cost around $37 million a year.

An independent engineering report to the Government on the feasibility of establishing a major port in the Manukau Harbour concludes it is technically feasible to open and maintain a navigation channel suitable for the size of ships serving a large-scale port - but there are plenty of red flags.

The report by Tonkin & Taylor and subconsultants for the Ministry of Transport follows 2015 and 2020 studies which highly ranked Manukau Harbour’s a potential as new large-scale port site for Auckland.

Tonkin & Taylor’s two-year study concluded that from an engineering perspective it would be technically feasible to open and maintain a navigation channel and the spatial needs of a large-scale port could be met, and navigational operability risks could be managed.

“While the Manukau Harbour is technically feasible, there are a number of significant risks relating to capital and operational costs, consenting and potential adverse effects to physical coastal processes.

“These risks may be possible to overcome and/or manage, but will likely present challenges in progressing.”

Further work could be taken to fully understand these risks, the consultants said.

The main findings of Tonkin & Taylor’s study included:

- Safe marine access to the Manukau Harbour could only be achieved with extensive and ongoing dredging which is shown to be feasible with the range of dredging methods that are available today.

- To form a navigation channel and port area, large volumes (70 to 90 M m3) of seabed material would need to be removed by capital dredging. This is estimated to take between four to six years to achieve and cost between $941 to $1244m with the configuration of the Manukau Bar at the start of the works dictating the equipment required.

- To maintain the navigation channel, very large volumes of accumulating sediment would need to be removed. This has been estimated to be in the order of 7.5 million m3 /year for the channel through the Manukau Bar and 0.7 million m3 /year for the inner harbour and port area. A dedicated maintenance dredger, capable of operating in the west coast wave climate, would need to be owned by the port and available 100% in winter and 50% in summer to achieve this. The cost of the dredger is estimated to be $176m, and the maintenance cost to remove this material is estimated to be $37m/year.

- Maintenance dredge volumes are very high when compared to New Zealand ports ... as well as international examples which are considered to have high maintenance dredging regimes.

- Material dredged through the Manukau Bar would need to be placed back in the active coastal system to avoid destabilising the balance of sediment over time. To achieve this, the dredger would need to place material in relatively shallow depths and additional steps are likely required, eg. partial loading, multiple handling with smaller dredgers, or alternative placement methods, all of which increase the complexity and cost. There is therefore a low confidence in the ability to manage this without adverse effect which is a high risk for a port in this location.

- Suitable dredged material from the inner harbour would be expected to be used to form the port reclamation. Our estimates show that there would be surplus material and unsuitable material that would need to be disposed of ... the ability to gain approval for this is therefore a critical risk to a port in this location.

Andrea Fox joined the Herald as a senior business journalist in 2018 and specialises in writing about the $26 billion dairy industry, agribusiness, exporting and the logistics sector and supply chains.