The Prime Minister is in Nelson visiting flood-affected areas today and speaking to impacted residents.

Nelson, Tasman and Marlborough have been devastated by a week of heavy rain, flooding and major slips.

Jacinda Ardern said the stories she's heard so far are devastating and there is potential for further slips, particularly with more heavy rain on the way.

Ardern will head up Maitai Valley/Nile St from 8.30am to 10am, before heading to the Trafalgar Centre to meet those affected by the flooding from 10am to 10.30am.

Ardern told reporters it was clear they would need an increase in the mayoral relief fund.

She said the relief fund allows for immediate needs to be met, but it will not be the quantum required for this recovery.

"It's devastating.... it's hard to imagine what that's like," Ardern said of the scale of the flooding.

"I expect that we'll be working together with local Government for some time to come but particularly with a focus with the roading network in the immediate aftermath of these events. There are isolated communities that we need to get back up and connected into the roading network."

Everyone Ardern has spoken to today, she said, had accommodation, "and we would like to make sure that remains the case".

She said the full cost will come over time, but they are there to support the local community and local Government.

A state of emergency remains in place, and there are still significant road closures in some parts of the region.

This comes as a shocking video emerged showing a large land slip progressing, and trees being dragged down by the moving earth.

The video was shared on Facebook and the user said the slip occurred in Marybank on Thursday evening.

More than 350 major slips are affecting roads and homes across the region, according to Civil Defence, the NelsonApp reports.

Controller Alec Louverdis spoke to displaced residents at Saxton Stadium yesterday and said the journey will be long for the region to recover, the news website said.

They noted as the scale of the clean-up starts to become clear, those affected have been touched by how the community has rallied around them.

MetService has issued a heavy rain watch for the Tasman region, west of Motueka from 1pm today.

Meteorologist Dan Corrigan said it's an area they're keeping an eye on after last week's extreme rainfall.

Scott Tambirasi helped clear this culvert of debris at night, as high rains hit Nelson. Photo / Supplied

He said while it normally wouldn't meet the criteria for a heavy rain watch, 20 to 40 millimetres could accumulate in some places around the Tasman district.

Corrigan warned there was a chance of more heavy rain in Tasman on Thursday - and they're continuing to monitor conditions.

In Christchurch, thick fog is expected to hang over the city until midday and MetService is urging motorists to take care on the roads as visibility drops to about 100 metres.

At least one flight has been cancelled from Auckland into Christchurch this morning.

Corrigan said a dull, grey afternoon is forecast once the fog clears, but it's here to stay all morning.

He says the fog will rise up off the ground and become low-cloud.