Rain has started falling in Roxburgh as dark clouds loom over the town still in clean-up mode from Sunday's downpour.

An Otago Daily Times reporter in the town said it started raining about 3.30pm as the forecast thunderstorms hit.

As the rain falls contractors are working as fast as possible to clear Reservoir Creek to prevent further flooding.

This comes after more than 40mm of rain fell in three hours on Sunday - almost as much as the area's average monthly rainfall for November.

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MetService said this morning there was a moderate to high risk of more heavy rain and thunderstorms for Central Otago and inland parts of North Otago and Dunedin today.

The Central Otago Disctrict Council said preparations were being made in case a storm affects the Teviot Valley again. This included bringing in a large supply of sandbags and having heavy machinery on standby.
"We recommend that you also make preparations, by having a bag packed with essentials for yourself and your family so that if you need to, you can leave in a hurry.

"If there is a need for evacuations the police will manage this. However, if you feel vulnerable or unsafe at home, you may want to arrange to visit friends or family for a few hours, if a storm develops."

A high police presence remains in the town and welfare was being provided at the Roxburgh Service Centre. Either call (03) 446 -8105 or drop in.

Contractors are working as fast as possible to clear Reservoir Creek to prevent further flooding. Photo / Tom Kitchin, Otago Daily Times
Contractors are working as fast as possible to clear Reservoir Creek to prevent further flooding. Photo / Tom Kitchin, Otago Daily Times

Central Otago sub area supervisor Senior Sergeant Ian Kerrisk said police were working with the council.

"We are planning for that eventuality. We have got to look forward, we know there is more rain forecast."

Extra police and firefighters are already in the Roxburgh area to help and advise residents.

Snr Sgt Kerrisk praised the strength of the community and how people were helping each other out. "The community is [full of] very resilient people."

Contractors were doing an "excellent job" trying to get the situation back to normal, he said.

Central Otago Mayor Tim Cadogan was in Roxburgh this morning, helping clean-up crews. He said they were working hard to minimise the risks before the heavy rain forecast for this afternoon.

"It's a high priority for Reservoir Creek [at the north end of Roxburgh near the evacuated houses] to be as clear as we can," Cadogan said.

"The forecast is for further heavy rain. We will get as much debris out of the creeks and culverts as we can but hopefully [the rain] won't come."

Cadogan said they were continuing to work on the water supply. It was still expected to be at least Wednesday before it was operating again.

"Other than that, it's just the general clean-up process," he said.

State Highway 8 through Roxburgh would remain closed for the time being, NZTA advised.

Meanwhile, Fire and Emergency New Zealand Southern Communications Centre shift manager Andrew Norris said "nothing significant" happened during the storm in Alexandra and Clyde last night.

Fire crew cleaned a few drains and reassured people. He said about five houses in Clyde needed minor assistance.

Thunderstorm warning

Today's thunderstorms may produce rainfall between 10mm to 25mm per hour and hail of 5mm to 15mm diameter, along with a moderate risk of severe thunderstorms producing localised downpours of 25mm to 40mm per hour.

A lesser risk of thunderstorms extends over a broader area including inland parts of Southland and Clutha.

They were also expected in the ranges of Nelson and Marlborough and the Canterbury High Country.

MetService meteorologist Tui McInnes said there was 38.4mm of rain in Alexandra between 7pm and 11pm on Monday night. In the first hour, Alexandra had 22.8mm and 13.2mm of rain in the second hour.

MetService issue a severe thunderstorm warning when there is 25mm of rain an hour. There were about 20 lightning strikes around Alexandra last night, McInnes said.

He said Wanaka had 13.2mm overnight, Queenstown had 1.8mm and Roxburgh had none.

What caused the Roxburgh deluge

MetService meteorologist Lisa Murray said Sunday's deluge was caused by cooler sea air flowing inland and meeting a ridge of high pressure over Central Otago.

The combination made the air more unstable and created a lot of afternoon convection.

''What you had over Roxburgh was cumulonimbus cloud - a thunderstorm cell - building up over the area. Because there was no wind around, it just sat there.

''Usually we have westerly winds that push the showers across and people get heavy rain for a little while, but it keeps moving east. But in this case, it didn't move. It just sat there for three hours and they got over 40mm of rain in that time.''

Because the ground was so dry and hard, it could not absorb the rain, which led to surface flooding.

The rain was accompanied by more than 450 lightning strikes in Central Otago.

Murray said the weather phenomenon was ''quite unusual'' for this time of year and it was possible the region could be hit by more thunderstorms in coming days - some of them severe with 25mm-40mm of rainfall in an hour.

''The area has all the ingredients for convection and thunderstorms, but exactly where they will pop up is very hard to predict.

''There is potential for this pattern to continue for the rest of this week, and if these highs persist, we could see more of this going into the rest of spring and early summer.''

McInnes said the weather was not unexpected, but it would usually be seen later on in the year.

"January and February are the months we expect this. It's more prone in the guts of summer. But this sort of weather isn't inherently unusual."

Tom Kitchin and John Lewis contributed to this report.
- Otago Daily Times