Hot summer days have set in and many of us are hunting out cool, airconditioned spaces — at home, at the movies or in shopping centres.

But for people like Daniel, that's not an option. He's lived on the streets for 20 years.

"Sometimes living on the street in summer can be really bad because no-one will let you in for a drink of water," he told News Breakfast.

"They assume you're going to use their toilet for drugs or whatever."

Daniel has been homeless for two decades. ( ABC News: Taryn Southcombe )

Throughout winter, appeals for blankets, warm clothing and food help those sleeping rough when the temperatures plummet.

But summer brings risks too, which is something Bill Crews from the Exodus Foundation in Sydney knows well.

"The average homeless person who eats with us has four major illnesses — and they're chronic illnesses," Mr Crews said.

"Any kind of fluctuation in the temperature causes enormous problems … a lot of the people we see suffer from the heat terribly."

Homelessness is a result of a number of complex issues and often chronic illness is a contributing factor.

Conditions like heart disease, respiratory illnesses and diabetes are often exacerbated by hot temperatures.

To make matters worse, some medications can make the effects of extreme heat more severe.

Mr Crews said when those who were homeless sought shelter in cool public spaces, like shopping centres or libraries, they were often moved on.

"Often homeless people have got bad body odours, so they'll empty a library," he said.

The Exodus Foundation works to help those living on the streets in summer by providing showers, deodorant and a cool place to sit.

Homeless people come for food and drink at the Exodus Foundation. ( ABC News: Taryn Southcombe )

Without a roof over their heads, those sleeping rough are also at risk from severe sun exposure, which can often cause heatstroke, sunburn and dehydration.

More than 100,000 people are homeless in Australia, according to the Australian Bureau of Statistics.

As the weather heats up, Mr Crews said there were some practical things people could do to help those sleeping rough, like donating light clothing, umbrellas, a drink of water or sunscreen.

"We run out of sunscreen, it's a big problem making sure everyone gets sufficient sunscreen," he said.

"People need to be treated with compassion; that's what we need."