Almost all of mainland Australia will be roasted in a huge heatwave next week, with the mercury likely to nudge 50 degrees in parts of the south.

Perth is in the midst of a four-day scorching as it plays host to the first day-night cricket Test between Australia and New Zealand.

Maximum temperatures are likely to near or exceed 40 degrees until Sunday, with the heat gradually shifting east.

Records are likely to tumble in many areas for days of consecutive heat even if individual daily maximums are broken, Kim Westcott, a meteorologist with Weatherzone, said. Remaining moisture in the environment may not fare well.