A motorscraper on its way to spread fill in steep gully near Cannons Creek.

It's the region's largest roading project. So, how is work on Transmission Gully progressing this wet winter? Damian George reports.

Wellington's recent wet weather has not only played havoc on the city's hillsides - it has also slowed construction of the Transmission Gully motorway.

The $850 million, four-lane Road of National Significance, which will run for 27 kilometres between MacKays Crossing in the north and Linden, Tawa, in the south, is still expected to be completed on time in April 2020, but wet weather is not making that easy.

MARK COOTE The motorway starting to take shape along Duck Creek.

The capital just recorded its ninth wettest July on record, causing more than 400 slips and some flooding in the region.

"Winter kicked off with a good start in June, but the recent wet spells in July have slowed down things a bit," project director Boyd Knights said.

READ MORE:

* Sneak peek at Transmission Gully progress

* Dismal summer has 'minor effect' on Transmission Gully

* Aerial photos reveal Transmission Gully progress

MARK COOTE A view looking west, with State Highway 1 on the left and the vast construction site on the right.

Together with the $630 million Kapiti expressway further north, and a project under way to solve congestion in Wellington central, it is hoped Transmission Gully will make the journey in and out of the capital safer and faster.

Described as one of the most technically challenging roading projects ever seen in New Zealand, work so far has already seen 2.2 million cubic metres of soil moved since construction began in 2014.

Almost 350,000 cubic metres has been moved this winter alone, with a total of 6.5 million to be cut before its completion. (To try and put some of that in perspective a 50m-long Olympic swimming pool can hold 2500 cubic metres.)

MARK COOTE The $850 million Transmission Gully motorway is on track for completion in 2020.

While not much can be seen of the road's progress while driving past the construction site on the existing State Highway 1, recent aerial photos reveal the road is starting to take shape.

A major milestone is expected to be achieved this spring, when 9500 cubic metres of concrete is expected to be poured to form the foundations of the two piers which will support the 230m-long, 60m-high Cannons Creek Bridge - one of 25 bridges needed for the project.

Once completed, four interchanges and two new link roads, in Waitangirua and Whitby, will connect the motorway to MacKays, State Highway 58 (Haywards Hill), eastern Porirua, and Kenepuru.

As of April, more than 2 million worker hours had already gone into the project, which Transport Minister Simon Bridges described as a "beast".

"One of the remarkable things about this is a lot of Wellingtonians, let alone New Zealanders, just won't have a sense that it's going on, because it's a bit out of sight, out of mind," Bridges has said.

Southbound traffic on State Highway 1 at Linden was moved to temporary lanes in the central median on Friday, so the regular lanes could be used for work on the Collins St overbridge, near Tawa.

The project, managed by the Wellington Gateway Partnership (WGP) has come a long way, but still has a long way to go.

MACKAYS CROSSING

Construction has yet to start here, with preparation work on soil still being done.

However, work is soon to begin on a bridge that will provide access to properties on the eastern side of the motorway, and preparation has started on installing a culvert to direct water from the Wainui Stream under the road.

Besides road building, environmental teams have also unearthed some "interesting artefacts" while digging for sand for the project from areas near the site. They include a suspected 19th century cannonball and a part of an adze, possibly from the 16th century.

WAINUI SADDLE

Earthworks are being done here 24/7, and work is also progressing on diverting the Horokiri and Te Puke streams.

A 2.5-kilometre access track between the area's work base, near the Battle Hill Farm Forest Park, and State Highway 58 is about half complete.

SOUTH FROM STATE HIGHWAY 58

Work is continuing on the bridges at Lanes Flat that will form part of the State Highway 58 interchange, while earthworks are progressing at the James Cook Dr interchange site.

Earthworks are also continuing from Lanes Flat to Takapu Rd, weather permitting, and bulk water mains that service Porirua are being relocated.

WHITBY AND WAITANGIRUA LINKS

Excavation work is being done for a bridge that will connect Waitangirua to the James Cook interchange over Duck Creek.

Work has also started on the Waitangirua link road, including clearing bush and creating access.

Underground services on Warspite Avenue, near the Niagara St intersection, need to be realigned before the link road intersection can be built.

That is expected to take a few months.

STATE HIGHWAY 1

The concrete columns motorists can see near Tawa are piles for bridges that will form part of the Linden and Kenepuru interchanges, where the motorway will join State Highway 1.

The new Collins Avenue bridge is being built in stages, with the existing bridge to be demolished. This work is mostly being done at night and SH1 traffic will not be affected.

Work is continuing to relocate a trunk sewer main beside the North City Tenpin bowling alley on Kenepuru Dr.