State Highway One over Northland's Brynderwyn Hills remains closed, after inspections today showed it was still unsafe to reopen.

The Transport Agency (NZTA) said it now planned to have the road open to all vehicles by the end of the week, weather permitting.

The NZTA's Regional Journey Manager Jacqui Hori-Hoult said they hoped to have one lane open today, but detailed inspections indicated the under-slip was bigger than they had originally thought.

"We have removed the loose soil and are starting work to stabilise this section of the highway that encroaches into the southbound lane hanging over an eight metre drop.

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"It's just not safe enough to open even just one lane to light traffic."

Teams were working in a challenging live slip environment from dawn to dusk to open that part of the state highway, as they were aware it was is critical to Northland, Ms Hori-Hoult said.

There are two alternative routes available for all traffic travelling between Northland and Auckland. People driving north are advised to use SH12 and the Paparoa-Oakleigh Road. Those heading south can travel through Waipu and Mangawhai.

The detours will add between 30 and 40 minutes to travel times.

Meanwhile, Mt Ruapehu has benefited more than other major skifields from an icy blast moving up the country.

MetService meteorologist John Law said there was a little more snow on the Rimutaka Hill Road throughout the day, however it was now easing off.

A severe weather warning remained in place for Taihape, and snow was falling on the Desert Road and would continue until early tomorrow morning, he said.

Mt Ruapehu had a nice dusting of snow, but a clear day down south meant little snowfall and cold temperatures.

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Southland and southern Canterbury had experienced rain today, and isolated showers had been reported in Northland and north of the Harbour Bridge in Auckland.

Otherwise the central South and North Island have seen plenty of clear sky.

A chilly southwesterly wind continued to hit Banks Peninsula, Wairarapa and Wellington, Mr Law said.

"This won't change too much. It looks like it will stick with us over the next couple of days and bring chilly temperatures."

The country will continue to shiver today as a south westerly flow ripples up the country, MetService says. Snow was already falling north of Wellington, over the Rimutaka Hill.