Just when you thought it was safe to go back in the water, taking a casual swim at your favourite beach just after a storm can pose a significant risk to your health.
If a significant amount of untreated stormwater has made its way to the ocean it could not only bring along a mixture of leaves, rubbish and oil but also sewage.
"One of the big problems that we have with our cities with stormwater is that our sewers are designed to overflow into the stormwater system," said Dr Stuart Khan, a water quality expert at the University of NSW.
"Very often in wet weather our sewers fill up, they spill over into the stormwater system and then the stormwater doesn't go to the treatment plant, it discharges onto the beach."
The NSW Office of Environment and Heritage recommends to avoid swimming at ocean beaches at least one day after heavy rain and for up to three days at estuarine swimming areas.
"It doesn't mean that all beaches are contaminated and unsafe, but if we are going to generalise that's a pretty good place to draw the line," Dr Khan said of the advice.
Checking water quality
Dr Khan said that with rain after long dry periods there was a first-flush effect when significant amounts of waste and debris could enter the stormwater system.
"If you see lots of discolouration in the water, lots of leaves and plastic bottles floating around, then that's a pretty good indication that there's stormwater," he told Wendy Harmer on ABC Radio Sydney.
"Where there's stormwater, there's bacteria.
"In other cases stormwater can look fairly clean and still have a very high pathogen and bacteria load.
"I'd be looking at rainfall as my key indicator [for contamination] more so than trying to measure water quality directly."
Dr Khan said beaches in enclosed bays were most heavily affected by pollution that ran into the waterways.
"They don't necessarily flush very well and get lots of clean ocean water coming in and replacing that contaminated stormwater once it has entered that bay."
Pollution forecasts issued by Beachwatch NSW report on possible water contamination levels at beaches around the Sydney, Newcastle and Illawarra regions.
Trouble in the sewers
Dr Khan said ageing sewerage infrastructure could often be infiltrated by stormwater seeping into broken piping which adds to the load handled by the system.
An additional problem he often saw was people diverting household stormwater into sewerage overflow relief gullies.
"It's illegal but not uncommon."
While urban development posed challenges for controlling stormwater drainage, Dr Khan recommended incorporating parks and gardens could help minimise the impact of stormwater runoff.
"If we have lots of concrete, lots of houses, roofs et cetera and very little backyard space, public gardens, places or opportunities for that water to infiltrate down into the ground, then we will have very rapid stormwater runoffs and we will have the impacts that we're talking about."
