ARARAT has been hit with a sudden downpour and thunder about 2.30pm Monday afternoon.

Bureau of Meteorology senior meteorologist Richard Russell said a severe weather warning has been issued for Victoria.

"We do have a severe weather warning current for other parts of the state where very isolated thunderstorms are highly likely to have heavy rainfall associated with them.

"These thunderstorms ... will deliver a lot of rain in a short amount of time."

Power outages continue to impact Horsham and Grampians customers, as well as another seven customers in the Streatham region south of Ararat.

View current weather warnings here.

Update, 12.30pm: A Powercor spokesman said crews were continuing work to restore power to customers impacted by storms over the past 24 hours.

"Widespread and severe lightning has damaged the network in many parts of the network, with 3200 customers currently off supply," he said. "There are currently 93 customers off supply in the broader Horsham area, plus 67 in Horsham itself.

"More severe lightning is expected to occur again today, with the storm system expected to peak this afternoon.

"In the event of an outage, stay clear of fallen powerlines and report them to us immediately on 13 24 12."

Prepare for Power Outages

Powercor released a range of tips to advise how customers can plan for outages in order to stay safe and minimise inconvenience.

Earlier: Severe thunderstorms in the Mallee are expected to produce heavy rainfall that may lead to flash flooding on Monday.

The Bureau of Meteorology has issued a severe thunderstorm warning for the Mallee and parts of the northern country.

Locations which may be affected include Mildura, Ouyen, Swan Hill, Kerang, Walpeup and Birchip.

This comes after a thunderstorm brought significant rainfall overnight on Sunday to parts of the Wimmera, as well as widespread power outages.

A spokesman from Powercor said 67 homes in Horsham were still without power on Monday morning.

The Powercor website said earlier customer outages totalled 3300 in Horsham alone.

There have also been outages reported in Minyip, Murtoa, Dooen, Wonwondah, McKenzie Creek, St Arnaud, Sheep Hills, Kewell.

Bureau of Meteorology Victoria senior meteorologist Richard Russell said the storm hit the Wimmera in the early hours of Monday morning.

Mr Russell said the two highest rainfalls were in Horsham with 31mm and Rupanyup with 33mm, while Longerong received only 4.8mm.

"That is just showing the nature of storms and how local rainfall can be," he said.

Mr Russell said there was a slight risk of more storms in the region today but that winds were the main concern in the coming days.

"On Wednesday there are strong winds developing and the potential for showers and storms," he said. "Then on Thursday a cold front will come through with more winds."

Country Fire Authority District 17 operations officer Lindsay Barry said the storm "was a bit wild" overnight and CFA crews had attended to four fires.

"There were two tree fires and two haystack fires," he said. "Where those fires had occurred there has been significant rainfall.

"Heavy equipment is on site to control them."

The fires on Sunday night were in Rupanyup, Bungalally and Lower Norton.

The CFA issued a community information notice for the haystack fire at Noradjuha-Horsham Road, Lower Norton.

The notice said that haystack fires were not likely to spread into grassland but do generate a lot of smoke, which would be visible from nearby roads and communities.

The fire presented no immediate threat to the community and requires no action.

Mr Barry said the CFA were preparing for the severe fire danger rating forecast on Wednesday.

"Any moisture at this time of year is more than welcome,' he said.

"We are working with Forest Fire Management Victoria to use water bombing aircraft to survey Little and Big Desert and the Grampians for fires.

"Just because it is wet on the ground doesn't mean there isn't a risk."

SES Horsham's Jarrod McLean said crews had only responded to one job in Horsham overnight, which was damage to a house.

"For the intensity of the storm we were pretty lucky, " he said.

Mr McLean said the SES also responded to a tree down at Miga Lake near Goroke and Edenhope.

READ MORE: 'Losing thousands of dollars': Grampians businesses 'blindsided' by decline in visitor numbers

While you're with us, you can now receive updates straight to your inbox twice weekly from the Wimmera Mail-Times. To make sure you're up-to-date with all the news from across the Wimmera, sign up below.