The Australian Medical Association is warning people to monitor their body temperature during this weekend's heatwave.

AMA president Richard Choong says people need to keep hydrated throughout the two days.

"We don't realise how hot it is because we are used to the warmer days and doing things we think we can cope with but the reality is it is a lot hotter than what we have experienced so far, so we need to take better precaution," Dr Choong said.

Severe to extreme heatwave conditions are forecast to spread through south-east Australia, with possibly no relief for inland areas for well over a week.

The Bureau of Meteorology (BoM) says heat building over recent days in Western Australia will migrate east through the weekend.

Melbourne, Adelaide, Canberra and western Sydney will all feel the heat, with temperatures in the high 30s to low 40s for at least four to five days.

The extreme heat has been building this week in parts of the Pilbara region, where temperatures eight to 10 degrees above the January average have been recorded.

Onslow airport recorded 48.7 degrees Celsius on Wednesday and 48.2 on Thursday.

The heat is spreading south to Perth, where the forecast maximum on Saturday is 44 degrees - which is 13 degrees above average - and 41 degrees on Sunday.

What you should do Read the ABC Emergency website about how you can prepare for and then deal with the sweltering conditions of a heatwave

As the heat is drawn east, forecast models are showing no signs of a clearing pattern developing until next weekend at the earliest.

Even then it is just the coastal districts of the south-east states that will get some relief with continuing heatwave conditions across the entire south-east inland.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Listen Duration: 3 minutes 19 seconds 3 m 19 s Extreme temperatures forecast for WA Download 1.5 MB

It has been a scorching start to 2014 for many parts of the continent.

A week ago, temperatures in parts of Central Australia, north-western New South Wales and Queensland approached 50 degrees, setting new records.

The sweltering conditions came as a BoM report revealed that 2013 was the hottest year on record in Australia.

Heatwave status can be forecast four days in advance

A new BoM heatwave forecast provides a national heatwave status up to four days ahead and includes three heatwave categories, providing an indication of how widespread the effects of the heat will be felt through the population affected.

This new product has resulted in a specific definition of heatwave conditions for Australia, which is now considered to be three days or more of high maximum and minimum temperatures that is unusual for that location.

Sorry, this video has expired Graham Creed explains the BoM's new heatwave forecast service

The heatwave forecast incorporates this definition and a heatwave intensity using the maximum and minimum temperature over a three-day period, the local climate averages, and how the temperatures have changed over the past 30 days.

The conditions in the past 30 days gives an indication on how acclimatised a population faced with higher-than-normal temperatures will be when heatwave conditions develop.

Extreme conditions is considered the highest level, with an impact across multiple areas such as infrastructure, transport, energy, agriculture and both healthy and vulnerable people are at risk of injury.

Extreme levels of heat will also coincide with dangerous fire weather conditions across the southern states.

More detailed information and the full forecast maps on the heatwave forecast can be found here About Pilot Heatwave Forecast and Heatwave Forecast for Australia.