If you're sick of the heavy rain, you better get used to it. The intense downpours and flash flooding Sydney and the Illawarra experienced over the past few days are going to become more frequent, a Sydney hydrologist says.

As the atmosphere heats up because of climate change, its capacity to hold moisture also increases, which means greater volumes of rain will fall during storms, said Fiona Johnson, from the School of Civil and Environmental Engineering at the University of NSW.

More rain in our cities will most likely cause havoc with traffic and public transport and may increase the number of people injured or killed in flash floods, such as the man who drowned after being swept in to a storm water drain at Lucas Heights on Monday.

With every 1 degree of warming, the amount of moisture in the atmosphere increases by at least 7 per cent, a relationship scientists have known about for 150 years, said Dr Johnson, the Herald's scientist-in-residence, a program organised by the Australian Science Media Centre.

But in the past decade, scientists have observed rainfall intensity increasing up to 15 per cent, a phenomenon called super-scaling.

"The reasons for this super-scaling are still not completely understood but it is thought that it is due to changes in the types of storms or possibly from extra energy available within the storm itself," Dr Johnson said.

On Monday, Bankstown received just under 50 millimetres of rain between 1pm and 2pm, the equivalent of 15 days' worth of rain falling in a single hour.

"This depth of rainfall would only occur on average one year in 50 years," Dr Johnson said.

But with an increase in temperature of just 1 degree, such intense rainfall may increase the rate of these dramatic storms occurring to once in 20 years in the future.

"The short sharp storm on Monday afternoon in Sydney is the type of event that is predicted to increase in intensity in the future," Dr Johnson said.

In cities dominated by concrete, the most obvious impact of higher rain volumes will be more flash flooding.

While engineers design stormwater pipes to cope with extreme downpours that have a probability of occurring once in every five to 10 years, an increase in rain intensity would most likely exceed the capacity of stormwater systems more often, Dr Johnson said.

"When the capacity of the pipes or the inlet grates is exceeded, then the excess water will generally flow along roads or wherever the easiest flow-path occurs," she said.