No fires can be lit in the open air between 12.01am and 11.59pm and the fire authority has advised people living in areas at risk of fire to activate their bushfire plan.

The Wimmera's fire danger rating for Wednesday is very high.

Country Fire Authority State duty Officer Greg Christison said conditions on Wednesday in the Wimmera will be very hot with temperatures reaching the low to mid 40s and winds of up to 30km/h.

"Tomorrow's conditions could make it difficult for firefighters to suppress a fire should one start, therefore we are asking people to take heed of the Total Fire Ban conditions," he said. "People need to be aware of the increased fire danger and ensure your fire plan covers all possible contingencies."

The Bureau of Meteorology has forecast very hot temperatures for the region, with Horsham set for a top of 43 degrees on Wednesday, 39 degrees on Thursday and 46 degrees on Friday.

Some of Horsham's Firebrace Street retailers will offer extended trading hours from Wednesday to give people a chance to escape the heat and the crowds with their Christmas shopping.

Horsham's Christmas in the Heart organiser Di Bell said the event on Thursday night would not feature any entertainment or a visit from Santa due to the extreme temperatures.

"The general consensus is that we want customers to beat the heat," she said.

Amcal Horsham owner Bianca Inkster said not having Santa visit on Thursday was "a bit of a dampener" in the lead-up to Christmas, but the pharmacy would have extended shopping hours.

"We will trade from 7am until 7pm on Wednesday, Thursday and Friday," she said. "We took the initiative to stay open longer to make it easier for people to support local business - especially those who find it harder in the heat like families and older people."

Wimmera Outdoors owner Gary Jelly said it was a "great thing" that retailers were opting to extend trading hours.

"It gives people an opportunity to purchase something before or after work," he said. "If the shops here don't give people that option to shop local, we face losing businesses here or people going to other places like Ballarat."

READ MORE: Three-day heatwave will bring extreme temperatures to the Wimmera

Wimmera Health Care Group acting director of primary and community care, Barry Arnott, said people should take precautions to avoid heat-related issues.

"Hot nights followed by hot days cause more health problems as they don't allow your body to recover," he said. "A health temperature above 32 degrees in Horsham means we have to take extra precautions to keep cool."

Mr Arnott advised people to wear lightweight and loose fitting clothing, eat light meals with not too much protein, and to regularly drink non-alcoholic fluids - even if they didn't feel thirsty.

"Once you become thirsty, you are already becoming dehydrated," he said. "Be aware of signs of dehydration such as darkening urine, light headedness, feeling unwell."

Smallaire sales consultant Rawley Whitworth said the business had been busy with people wanting to get their cooling products serviced ahead of the heatwave and Christmas.

He said evaporative cooling units should be left on all day and overnight where possible during hot weather.

"It has to work harder if it is being turned on and off," he said.

Mr Whitworth advised people to ensure their air-conditioner was in good working order by checking the pads on the unit for water drips or if they were cracked or crumbling.

"The best temperature for a refrigerated air-conditioner is 24 to 25 degrees," he said. "That will ensure it doesn't go into overload."

Powercor network safety and bushfire mitigation manager, Dene Ward, said extreme weather put additional pressure on the electricity network and could cause outages. He said bushfires could also affect electricity supply.

"Over 1000 people across our 14 depots in Western Victoria have been working throughout the year to get the network ready for summer. Making sure we continue to deliver safe and reliable power to the community is our priority," he said.

"If the power does go out, we are ready to respond quickly, and we encourage Western Victorian residents to be as prepared as possible."

Mr Ward said people should prepare contingency plans if they relied on life support equipment or needed electrical items for the elderly, pets, babies or people with a disability.

He said people should consider alternative power sources for water pumps and cooking equipment, and make sure their phones and laptops were charged so they could stay connected.

READ MORE: Wimmera CFA and SES units receive grants for new equipment, vehicles

"There is so much information available about how to plan and prepare for a bushfire emergency that it's easy to forget about what impacts a power outage can have, too - like causing ATMs to go down," he said.

Horsham Aquatic Centre manager Sam Winter said the Horsham War Memorial Pool would operate its regular summer opening hours and extra lifeguards were rostered to cope with an expected increase in patrons.

"There will be three lifeguards on at a time to ensure they get regular breaks inside," she said.

Ms Winter said visitors to the pool could bring their own shade equipment such as umbrellas and day tents, due to the limited amount of cover at the facility.

"We have sunscreen available to purchase at the front desk as well," she said.

Horsham Rural City Council operations manager Rob Moir said council workers - such as those involved in construction, road work and gardening - would work in a way that avoided being outside in the worst of the heat.

"Outdoor staff start early in the morning and typically would focus on asset inspections or equipment maintenance later in the day in extreme heat," he said. "We also have graders and water tankers on standby around the region should they be required in an emergency.

"Ultimately, staff do work through hot weather where it is safe to do so and we keep them hydrated with water supplements and icy poles."

Ingenia Gardens Horsham community manager Barb Cavanagh said residents would be able to beat the heat with air-conditioning in all units.

"We have also invited residents to gather in the community room where they can keep cool and enjoy some icy treats," she said.

Business Horsham were contacted for comment.

For emergency information in Victoria download the VicEmergency app, visit emergency.vic.gov.au, phone the VicEmergency Hotline on 1800 226 226 and follow VicEmergency on Facebook and Twitter.

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