The central North Island could also see snow by tomorrow night down to 800m, meaning the Desert Rd could be affected.
MetService meteorologist Tom Adams said people had called in with reports of snow to about 200m above sea level in Clinton, Lumsden and Naseby in the far south.
"Following that front overnight there is a lot of cold air and showers behind it. When you get that combination you can get snow to low levels."
Initially they were getting reports from people with snow on their high-country farms, but as the skies cleared they expected to see more snow on the ranges.
The show showers were forecast to ease during the day, before another front moved in tomorrow.
"We are in a very changeable pattern," Adams said.
Snow could fall to 400-500m in parts of the South Island, and potentially in Queenstown.
Heavy snow - 20-40cm - is expected above 400 metres in Fiordland, Westland south of Haast, Southland north of the Mavora Lakes and the western ranges of Otago south of Mount Aspiring during Thursday. More is forecast above 600m.
Milford Rd (SH94) closed overnight because of snow. The road was reopened this morning but significant snow was forecast for the area tomorrow, which could mean an all-day closure.
Road snow warnings also remain in place for Lewis Pass (SH7) and Arthurs Pass (SH73).
Road users are being urged to take extreme care due to the icy conditions.
Queenstown Lakes District Council said "Jack Frost has been working his mischief" and there was a high chance of ice across roads in the district.
Black ice had been reported in the Cardrona Valley and over the Lindis Pass and ice on the Crown Range, which had been gritted.
Grit trucks had been busy across the rest of the district and had gritted roads from Arrowtown through to Queenstown, up Fernhill and on Glenorchy Rd.
NZTA has issued a caution notice for black ice on SH8 between Tarras and Omarama.
Alexandra was sitting at a crisp -2.4C at 5.30am under clear skies, and Gore and Invercargill were at 4C with heavy showers and brutal southwesterlies.
Invercargill and Dunedin are in for a day of sleety showers and highs of 7C and 8C respectively. Most of the South Island will spend the day in the low teens.
UPDATE: 4.30pm & it's STILL dumping!! 40cm+ since 6am and some massive drifts around the mountain. Let's see where things end up once it all clears but last time we checked we lost the ruler!! YEEEEWWWW!! ❄️⛄💯
Posted by Cardrona Alpine Resort, NZ on Monday, 21 May 2018
The southern skifields are celebrating the early arrival of winter.
Cardrona, Coronet Peak, the Remarkables and Mt Hutt are all reporting decent amounts of snowfall. By 4.30pm yesterday 40cm had already fallen at Cardrona.
