A massive slip has closed the main railway freight line between Auckland and Northland.

The slip brought down about 35,000 cubic metres of debris onto a section of road and rail line near Tahekeroa between Helensville and Wellsford, covering an area nearly three times as big as a rugby field.

The North Auckland Line between Swanson and Whangārei - one of the main routes for dairy products and cement to the rest of the country - is now expected to be closed for months, stopping all rail freight services to and from Northland.

Hundreds of freight containers will now have to be trucked on Northland's already fragile roads each week.

It was one of a number of KiwiRail lines expected to remain closed for the coming months.

KiwiRail chief operations officer Siva Sivapakkiam Photo: RNZ/ Rayssa Almeida

Line to Northland

KiwiRail chief operations officer Siva Sivapakkiam said 50 slips fell on the rail line between Swanson and Whangārei, and fixing it could take up to three months, weather dependent.

"Our track teams have been busy clearing vegetation, repairing damaged rails and replacing culverts and we are still working through detailed engineering assessments for significant slips and flood damaged bridges," Sivapakkiam said.

"It's gonna be quite challenging ... one of the first things we really want to do is build back the rail track because it's one of the lifeline connections for us to move freight in and out of Northland."

"So, 10 weeks is going to be pretty challenging for us, and today's probably the first day we've gotten there," he said.

KiwiRail upgraded the North Auckland Line between Swanson and Whangārei in 2020 to make the track more secure.

Sivapakkiam said with the slips, the project would now have to start again from scratch.

"At the moment we haven't been moving a hell of a lot because the line has been upgraded. We started on the 27th of January, and with ... what's happened. We've now gone back to square one."

Regarding the slip at Tahekeroa, a large, temporary earth bund would be built so the slip could be cleared safely, then a permanent earth bund and significant drainage improvements would be built to help prevent future slips, he said.

More than 40 extra freight trucks per day will be on the already fragile roads of Northland after a major slip on the region's main rail network. Photo: RNZ / Rayssa Almeida

Hawke's Bay

The line between Woodville and Hastings was expected to remain closed for at least 3- 4 weeks.

"Once open it will allow us to then road freight to and from Napier," Sivapakkiam said.

"Track repairs are making good progress, detailed inspections of bridges are progressing, and we are setting up the temporary road-rail container terminal in Hastings."

It would be months before the section of line between Hastings and Napier would reopen, he said.

The line was badly damaged during Cyclone Gabrielle with a number of bridges damaged.

Bridge 217, north of Clive, lost piers and spans.

Napier - Wairoa track

There were a number of washouts and slips of various sizes on the track between Napier and Wairoa.

While high-level assessments have been undertaken via helicopter, ground level track inspections would begin this week.

"Given access to the rail line is difficult in some areas, inspections will take time," Sivapakkiam said.

"Our focus has to be on reopening the rail line to Hastings and then Napier, so at this stage Napier to Wairoa, which is used to transport logs, is expected to remain closed for some time."

Slips have badly damaged rail corridors across many places in the North Island Photo: RNZ/ Rayssa Almeida

More trucks and trips needed

Rail and Maritime Transport Union spokesperson Todd Valster said the closed rail lines and increased truck movements to shift what would normally be carried by rail could mean more stress to the already fragile roading system in Northland.

There would normally be one freight train through the region each day, with goods mainly heading toward Auckland.

"The goods now have to be moved by truck, but there are big challenges with the roads as well - not just rail, you're looking at potentially 24 to 48 trucks a day," Valster said.

Transporting New Zealand chief executive Nick Leggett said there would be a significant effect on the roads.

"What goes on a train still has to be transported and then will have to go on the back of a truck - so it will will mean more trucking movements," Leggett said.

One of the company that relied on the North Auckland Line was Fonterra, which said it had switched its product transport from rail to road.

Fonterra global supply chain director Anna Palairet said using trucks as an alternative to trains was less efficient and more carbon intensive.

KiwiRail said works to repair the road and rail line will begin this week and will include building a temporary earth bund to make the site safe before beginning to remove the slip.

It said its working on a long-term engineering solution to avoid further slips.