Dublin Street Bridge - business case development

• Project typeBusiness case development
• Project valueAn estimated $2 million (for business case)
• Project scheduleThe business case is expected to take at least two years to complete

Dublin Street Bridge is a vital connection between central Whanganui and the eastern suburbs via Anzac Parade (State Highway 4), providing a safe, effective and efficient link to essential services, business and trade commute, and schools in a growing Whanganui city.

As the bridge spans the Whanganui River, the council is committed to placing the awa at the centre of the business case development, through Te Awa Tupua (Whanganui River Claims Settlement) Act 2017 which conferred legal personality on the awa (the awa is legally recognised as a person). The council is partnering with Iwi and Hapū and the business case was introduced at a high level to Hapū at a recent Wai Forum.

Current focus

The 309m through-truss bridge, which has separate cycling and walking paths on each side, has served the Whanganui community for more than a century. It is now approaching the end of its designed service life, with extended maintenance needed.

Its carrying capacity has already been downgraded from Class 1 (44 tonnes) to 6 tonnes after a structural assessment and reduced lane width, significantly limiting its use for modern traffic and freight. A new bridge would be expected to be Class 1 rated, able to carry truck and trailer units, reducing the heavy traffic travelling via the City Bridge and to provide more resilience during emergencies.

The council has started working with NZ Transport Agency Waka Kotahi (NZTA) to begin work on the feasibility and funding of a bridge replacement, with initial work focused on the bridge's current state.

A Point of Entry business case has been submitted to NZTA, as part of a requirement to establish strategic fit. This has been accepted by NZTA and we are now moving to develop a detailed business case that is expected to take at least two years, given the scale of the project, its complexity and the specialist information that needs to be gathered. The business case will explore viable options (including replacement or remedial work) and is a step in the process to try to secure funding and ensure the best outcome for the Whanganui community, now and into the future.

Surveying, data collection and other investigations are underway for the business case, which at a later stage will involve collaboration between various stakeholders.

It is important to note that if a case for replacement is successful, there are several steps including design and consultation that need to be completed before any physical work on a replacement begins (recognising this will take some years). 

Data collection update – November 2025

A detailed aerial and hydrographic survey of the Dublin Street Bridge and the surrounding area of the Whanganui River has been carried out to help the council and NZTA better understand both the land and the riverbed as part of the business case process. To do this, we used two types of technology, provided by WSP.

The ground surface above water level was obtained using a drone with a LiDAR scanner that flew over the site. This allowed us to capture highly accurate surface measurements, and to see how the terrain changes around the bridge.

The hydrographic survey, to map the surface under water, was done with a bright yellow unmanned survey vessel – a small remote-controlled boat – that travelled along the river. It used a multibeam echo sounder, which sends sound waves through the water, to map the shape and depth of the riverbed. This groundbreaking technology delivers precise readings and measurements.

By combining information from both the drone and the survey vessel, a comprehensive 3D model of the land and riverbed has been developed, showing what they look like above and below the water. This detailed picture will support engineering assessments of the current bridge, design, and planning for the next stages of the business case. See the images below.

More about the new technology

WSP is the first organisation in New Zealand to gain Maritime New Zealand approval to operate remotely controlled survey vessels – the Apache 3 and Apache 4. This achievement followed months of application and compliance work to ensure they could be used in rivers, lakes, harbours, and coastal waters.

The Apache 3 can carry out autonomous bathymetric surveying with Global Navigation Satellite System (GNSS) and Inertial Measurement Unit (IMU) precision, while the Apache 4 takes capability to the next level, using multibeam sonar for high-resolution underwater mapping.

More about the new technology from WSP 

Funding and the Long-Term Plan 2024-2034

The replacement of Dublin Street Bridge has been identified as a priority in the Long-Term Plan 2024-2034, with an indicative figure of $69 million but a price estimate depends on the solution chosen.

The bridge's replacement relies on collaboration with NZTA and/or other central government funds. Usually, NZTA will fund 62 percent of the total cost, leaving 38 percent for the council to fund. The council's share of the cost will be funded via loans to not only spread the cost over a longer time period but also reduce the impact on future rates.

The business case process

NZTA has developed guidance for councils developing business cases for investment through the National Land Transport Programme (NLTP). Any request for funding from the NLTP must be supported by a fit-for-purpose investment proposal that uses the principles of the business case approach (BCA).

As a principles-based approach, the BCA involves very few strict requirements. However, there are three critical steps that are required when developing a business case. What changes from one business case to another is the level of detail needed to complete each step.

The three critical steps are:

• the case for change
  (the strategic context is defined, problems and benefits developed, along with investment objectives and risks and uncertainties assessed)
  
• optioneering
  (the economic case is developed e.g. identifying alternatives and options, strategic response and finally deciding on the preferred option)
• refining the preferred option.
  (the commercial, financial and management case is developed)

Read more about the process here

Image above: The Business Case Approach process - the red arrow points to the stage the council is at in its business case development

From the past

Image below: Driving in the first pile for the Dublin Street Bridge, Wanganui. Tesla Studios:Negatives of Wanganui and district taken by Alfred Martin, Frank Denton and Mark Lampe (Tesla Studios). Ref: 1/1-017352-G. Alexander Turnbull Library, Wellington, New Zealand./records/22865581

Image below: Tram crossing Dublin Street bridge in Wanganui. Tesla Studios:Negatives of Wanganui and district taken by Alfred Martin, Frank Denton and Mark Lampe (Tesla Studios). Ref: 1/1-017383-G. Alexander Turnbull Library, Wellington, New Zealand./records/23041458

Image below: Workmen and roller on Dublin Street bridge, Wanganui. Head, Samuel Heath, 1868-1948 :Negatives. Ref: 1/1-008198-G. Alexander Turnbull Library, Wellington, New Zealand. /records/22726615