The hydrofraise machine is doing excavation work for Watercare’s $1.2 billion project to clean up Auckland's waterways.

A mega digging machine has arrived in Auckland to help clean up the city's beaches.

Construction of Watercare's $1.2 billion central interceptor project is in full-swing with a hydrofraise machine arriving to do the heavy lifting.

The 14.7 kilometre long, 4.5 metre wide tunnel is the biggest wastewater infrastructure project in New Zealand history, with the project due to be completed in 2025.

The project aimed to clean up the city's waterways by transporting wastewater from central Auckland to the Mangere Wastewater Treatment Plant.

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Watercare's Central Interceptor executive programme director Shayne Cunis said the 38-metre high hydrofraise machine was being used to excavate a 50-metre deep structural support at Greenwood Rd, Māngere.

The crane is visible from streets bordering the site and is believed to be the largest ever seen in New Zealand, according to a Watercare statement.

"This is a very complex, highly specialised form of construction and we've sourced experts from around the world to operate this machinery," Cunis said.

Preparatory work was also underway at two other sites in Mt Roskill and Hillsborough as part of the project's construction.

A giant tunnel boring machine was being assembled in China for the project and would arrive by boat towards the end of 2020.

Supplied The 38-metre high hydrofraise machine, which is now working on Watercare’s $1.2 billion Central Interceptor project.

Watercare chief executive Raveen Jaduram explained in 2018 the reason for the central interceptor.

"Parts of the old Auckland City Council area have no stormwater system, so when it rains the stormwater goes into the wastewater pipes and then overflows into streams and beaches," Jaduram said.

"It was designed to do that in the early 1900s because it was acceptable back then – and with fewer houses, there was more land for the rain to soak into. But Aucklanders' expectations have changed – we want a clean and swimmable water environment."

The project was expected to reduce the volume of overflows by at least 80 per cent and reduce the number of times each year that contaminated stormwater is dumped into the harbour from 52 days a year to 10 or less, according to Watercare.

Construction of the central interceptor began in December.