If there is one place that will really feel the heat on Thursday, it is western Sydney.

With most of the city expected to heat up by 10am, the west is expected to be as hot as 40 degrees by lunchtime.

"I couldn't say specifically which suburb will get the highest temperature at this stage, but probably most of Sydney will be in the high 30s and the western suburbs could touch 40 by the middle of the day," said Bureau of Meteorology forecaster Christopher Webb.

The cool change is unlikely to reach the city until 5-6 pm, and will bring "quite a drastic change" in temperatures and rain when it does arrive, said Jake Phillips, another bureau forecaster.

Before then, the city could get "a touch hotter" than the 38 degrees predicted by the bureau, bringing it closer in line with hot spots in Sydney's west. "It looks like we won't get a sea breeze" ahead of the change, Mr Phillips said.

In the meantime, visiting air-conditioned spaces, drinking water, staying out of the sun between 11am and 5pm, insulating the home and using light-coloured building materials are all methods recommended by western Sydney councils to combat severe heatwave.

Heat expert and president of the Climate and Health Alliance Elizabeth Hanna said while people understand that elderly and young children are at risk, many fail to realise that we are all vulnerable to heat.

"We know that people don't drink enough, and our new research shows workers significantly overestimate their capacity to keep working in the heat, meaning they're not as cautious as they need to be."

Dr Hanna added that pet owners should be prepared for heatwaves at home and have a cool place ready for their pets, such as a laundry or shady part of the backyard.

'Heat island'

The effects of intense heat have not escaped the gaze of western Sydney councils, which are expected to experience between five and 10 additional hot days by 2030 due to rapid rates of construction.

Over the next 15 years, 180,000 homes will be built in western Sydney, a scale of growth likely to exacerbate an "urban heat island effect".

"Urban heat is an issue for all cities, but western Sydney's unique geography and lack of sea breeze means the region is already much hotter than its eastern counterparts," said Councillor Tony Hadchiti, president of the Western Sydney Regional Organisation of Councils (WSROC).

New designs for heat-resistant bus shelters, tree-planting programs and aerial heat mapping are some of the initiatives local councils are exploring in an attempt to design communities that are heat resilient.

The Parramatta heat maps have assisted in identifying priority hot spots and will inform the delivery of better infrastructure through improving public space design to reduce summer heat.

On average, Parramatta experiences around thirteen days per year which are 35 degrees and over, compared to Sydney which averages around four.

The City of Sydney has developed a heatwave response plan in its new climate adaptation strategy, by planting trees and creating additional open space to withstand extreme temperatures.

For those planning to hit the beach on Thursday, Surf Life Saving NSW has issued a reminder to take care and avoid the sun during the hottest parts of the day.

"All our volunteers and lifeguards are well trained in first aid, so if you suffer any symptoms of heat stress, even if you think it's minor, head to the shade and the lifeguards and seek help."

Asthma alert

If the heat wasn't enough to cause discomfort, high pollen counts may also add to health concerns.

NSW Ambulance on Thursday was urging asthma sufferers to ensure they have a plan to manage their condition.

Warmer weather heightens the risk of asthma attack, allergies and hayfever for many sufferers, the service said in a media release.

"A serious asthma attack can be fatal in only minutes, so it is absolutely vital asthma sufferers carry their medication with them at all times, and call Triple Zero (000) and ask for Ambulance  should their symptoms become severe," Joel Bardsley, said.

The change should usher in a few days of relief from the heat, with maximums in the city likely to remain in the low to mid-20s until Tuesday, the bureau said.

With Peter Hannam