The Bureau of Meteorology has introduced a new national warning system for severe or extreme heatwave conditions.

The Heatwave Warning service will alert the public that a heat hazard is forecast within the next four days, helping Australians to prepare for and lessen the impact.

“Heatwaves are Australia's most dangerous environmental hazard, resulting in more community deaths than bushfires, floods, cyclones and severe storms combined,” BOM said.

Alerts will be issued at 3pm through the BOM Weather app and website, and via media, for all states/territories except WA, which will be alerted at 2pm local time.

The warnings carry five sets of action statements which were agreed with health and emergency service agencies across Australia. Heat health messages will also be provided by state and territory agencies.

A heatwave occurs when high and low temperatures are unusually hot over a three-day period at a location. A successful trial of the alerts found they helped agencies provide timely information of the health risks of extreme heat and better equipped health departments to assist those vulnerable to heatstroke.

The Heatwave Warning includes colour-coded maps and a description of the expected high and low temperatures, when the heatwave will peak and affected towns and communities.

BOM compares the forecast maximum and minimum temperatures for each three-day period in the coming week to what would be considered hot for that location, and also to observed temperatures over the last 30 days.

“This calculation considers people's ability to adapt to heat. For example, the same high temperature will be felt differently by residents in Perth compared to those in Hobart, who may not be used to the higher range of temperatures,” BOM said.

A sharp and sustained rise in temperature after relatively cool weather in a location will also show up in the heatwave forecast, and BOM says temperatures that meet the criteria for a heatwave at the end of summer will generally be hotter than the temperatures that meet it at the beginning of summer.

Children and people aged over 65, living alone, socially isolated or on some medication are among those most affected by extreme heat. Signs of heat stroke are fits, confusion and staggering. It is a life-threatening emergency and can cause a person to collapse or fall unconscious.

Heat exhaustion symptoms include headache, body temperature above 40 degrees, muscle cramps, exhaustion, nausea, dizzy spells, pale, cool, clammy skin becoming flushed, and a rapid but weak pulse.

The Red Cross says those affected should lie down in a cool or shady area, remove or loosen clothing, fan and moisten skin and stretch affected muscles to ease pain.

If the person is unable to drink, vomiting, unresponsive or does not improve, call an ambulance.

Heat stroke is even more serious. It means the body is no longer able to regulate its temperature via sweating, and organ damage can occur.

Sufferers typically no longer sweat, have hot red skin, a high temperature, a rapid and strong pulse and rapid, noisy breathing, irrational or aggressive behaviour and a deterioration of consciousness.

After calling an ambulance, cool the person using a wet towel or sheet and a fan, or use ice packs around the neck, groin and armpits. If shivering occurs reduce active cooling.

“In the last 200 years, severe and extreme heatwaves have cost more lives than any other natural hazard in Australia. Heatwaves affect healthcare, transport, emergency services, energy, agriculture and many other sectors,” BOM said.