The South Australian government will activate a code red response from 5pm on Friday, with the Bureau of Meteorology predicting temperatures of more than 40C across much of the state in the next few days.

The code red will make additional services available to people sleeping rough, including daily outreach visits by specialist homelessness services.

"Help is available to the vulnerable, isolated and elderly and I urge anyone who may be at risk to access the services on offer, and for their family and friends to continue to keep an eye on their welfare," Human Services Minister Nat Cook said.

The WOMADelaide festival, which starts on Friday evening at Adelaide's Botanic Park, has placed bat bins around the site so staff can dispose of any flying foxes that succumb to the heat.

The world music and arts festival is one of several events set to be affected, with Adelaide in the midst of the busiest period on its festive calendar.

Fringe Festival, the South Australian athletics championships and the Adelaide Cup horse race are all scheduled to take place this long weekend and have heat management plans in place.

Greyhound Racing SA and Harness Racing SA have rescheduled several races due to the heat, but the RSPCA was concerned about horse races still scheduled to take place in forecast temperatures of up to 42C.

The bureau has forecast Adelaide will hit maximums of 40C on Saturday, 38C on Sunday and 38C on Monday.

The heatwave is being brought about by a high-pressure system in the Tasman Sea directing hot, northerly winds across South Australia, senior meteorologist Jonathan Fischer said.

The SA State Emergency Service urged revellers enjoying the festivities to stay cool, hydrated and sun safe.

"Heatwaves cause more deaths in Australia than any other natural hazard," SES chief of staff Derren Halleday said.

Mr Halleday warned people to take care around trees, which are more likely to shed large branches when they suffer from heat stress in high temperatures.

Chief Public Health Officer Nicola Spurrier advised people to go to a shopping centre or a library if they were unable to keep cool at home.

"The heat affects everyone but certain groups such as babies and young children, pregnant women, the elderly and people who have chronic health conditions are more at risk than others," she said.

Heatwave warnings are also in place for Victoria, Tasmania and southern NSW.

Melbourne is set to reach temperatures of 39C on Saturday and 38C on Sunday, while extreme fire danger warnings have been imposed for Victoria's central, south west and north central districts.

Tasmania Fire Service deputy chief officer Matt Lowe urged campers to stay fire safe, with high fire danger ratings for Saturday imposed across most of the state.

Australian Associated Press