Liverpool has experienced temperatures in the early to mid-30s this week but after this afternoon's predicted thunderstorm, temperatures are expected to drop to 25 tomorrow and 24 on Friday.

The maximum temperature for Liverpool this year was 38.8 on Saturday.

The highest January temperature on record for the area was 44.8 in 2003.

With the hot weather have come health warnings from the Sydney South West Area Health Service.

Stephen Conaty, acting director of public health, said heat-related illness can be severe.

``It can range from mild, such as rash or cramps, to serious medical emergencies such as heat stroke or heart attacks,'' he said.

``Symptoms of heat exhaustion include nausea, faintness, dizziness, loss of appetite, weakness, headache and vomiting.''

Dr Conaty said anyone can be at risk of heat-related illness but those most at risk are the over-65s, babies and children, people with high blood pressure or heart disease, people who live in hot houses or flats, work in hot environments or attend outdoor events such as a sports carnivals or festivals.