The WA Health Department has issued a heatwave warning for Perth for the first time since 2016 as the city braces for a run of record-breaking heat until Sunday.

Key points: Three days of 40 degrees Celsius are forecast in Perth from Friday to Sunday

Three days of 40 degrees Celsius are forecast in Perth from Friday to Sunday Perth has never had more than two days of 40C or above in December

Perth has never had more than two days of 40C or above in December The extreme heat coincided with the start of the first cricket Test against NZ

Daytime temperatures are expected to hover close to 40 degrees Celsius for the rest of the week and the nights will offer little relief, only dropping to the low- to mid-20s.

Chief health officer Andrew Robertson has urged the public not to underestimate the effects of severe heat.

"There is a risk in any heatwave that we will get some serious illness," he said.

"Unfortunately we will probably get some deaths in this heatwave — they tend to be in the elderly or those with chronic disease.

"It becomes very critical that we do look out for our neighbours and our relatives who might be impacted and make sure they're managed.

"If they're becoming unwell get them to seek medical care early."

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Dr Robertson said there were a number of precautions people could take to avoid heat stroke, including staying hydrated, wearing light clothing, avoiding exercise in the middle of the day and limiting alcohol consumption.

"There is often an assumption that we're West Australians, that we know what it's like to have a hot day and that we'll be fine — these days have a major stress on all of the population and it's not just the high maximum temperatures, it's also the high minimum temperatures," he said.

The forecast heatwave comes on the back of Perth recording its hottest first week of summer on record. ( ABC News: Jon Sambell )

"I think most people accept that they don't sleep as well, they get exhausted and they find that their bodies are less able to deal with the impacts of the heat."

The Health Department, which is responsible for managing heatwaves in WA, would liaise with infrastructure providers to ensure planned water and power outages were delayed, as well as hospitals and St John Ambulance to ensure they were adequately resourced.

Records melt away as heat hits trains

Perth has never had more than two days of 40C or above in December, but that record is about to be challenged.

"In December, the highest number of 40-degree days we've ever recorded is two and the highest number of consecutive days that we've recorded is two so if we get to three out of this run, we'll break both the highest number of 40 degree days for December and also the consecutive number of 40-degree days as well," Bureau of Meteorology spokesman Neil Bennett said.

"Many parts of the west coast area, into the Wheatbelt, up to the Central West and down to the south coast are also going to see temperatures that will approach December records, so this is a widespread event."

The night time will offer little relief with minimum temperatures only dropping to the low and mid 20s. ( ABC News: Margaret Burin )

The forecast heatwave comes on the back of Perth recording its hottest first week of summer on record.

Currently it is tracking more than five degrees above its mean December maximum of 29.1C.

The city usually gets about five days above 35C for the entire month, but by the end of this week — a fortnight into December — Perth will have notched up 11 days above 35C.

Trains in Perth are slowed by up to 20 kilometres per hour when track-level temperatures reach high 30s. ( ABC News )

Train passengers have been advised to allow extra travel time over the coming days as temporary speed restrictions may be imposed on the Transperth rail network due to the heat.

Transperth said it reduced the speed of trains by about 20 kilometres per hour when track-level temperatures reached 37C on the Fremantle, Midland and Armadale lines and 39C on the Mandurah, Joondalup and Thornlie lines.

If track temperatures reached 41C, train speeds reduced by a further 10kph.

Extreme heat to test cricketers

The heatwave conditions will also inject an element of the unknown into the first Test between Australia and New Zealand starting at Perth Stadium on Thursday, with curator Brett Sipthorpe anticipating extreme temperatures at ground level.

He said how the wicket would behave under such extreme temperatures was unclear.

"It will be more consistent, hopefully, than last year, it's a stronger grass cover than last year so we're hoping for a bit more of a consistency, and then traditional slowly aging," he said.

"But the unknown is what the temperature is going to do. It could be 50 degrees out here."

The pitch had been watered regularly to help keep it together and prevent cracking and Sipthorpe said the moisture in the ground would make the wicket play slower when play begins on Thursday afternoon.

The Perth Stadium pitch offered some curly deliveries last summer, as Usman Khawaja found out. ( AAP: Richard Wainwright )

"I don't want it to go cracking open too quickly," he said.

"I expect it will be a little slower early on, but then it will pick up pace.

"It was one for a hundred last year and then after lunch all hell broke lose. I expect something along those lines.

"We haven't seen any cracking at this point which is really good, whereas last year we had cracking showing early on, so we're a lot more comfortable with how the preparation has gone."

No repeat of 'dangerous' MCG pitch

Scrutiny over the preparation of Test wickets has intensified following last weekend's abandonment of a Sheffield Shield game at the MCG, which was caused by extra moisture in the pitch causing divots that became dangerous as the ground dried.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 1 minute 34 seconds 1 m 34 s Sheffield Shield match suspended due to unsafe MCG pitch (Pic: AAP)

Sipthorpe said he wasn't concerned about a similar occurrence happening in Perth.

"All we can do is serve it up, so it's got the best chance of getting through five days of being a good cricket pitch," he said.

"Once they start [playing] we can't control what happens. We've done what we can to keep the moisture in and we've got good base compaction so it should bounce, it should have good pace."

Last year's wicket for the Test against India received praise from legendary batsman Sachin Tendulkar, who took to social media to criticise the average rating handed down by the ICC following the match.

Perth weather forecast