Sewerage treatment plants at Bondi, Cronulla, North Head, Wollongong and Malabar are among those that are frequently put on ‘‘bypass’’ during sustained wet weather, where partially treated sewage is pumped out with excess storm water.

Pollution warnings were given to swimmers at all Sydney beaches last week. However, the public is not always informed in time. Beachwatch put out an online public alert on July 1 to say the Warriewood plant, which services the Northern Beaches, was on partial bypass on June 29.

On 82 occasions from 2011 and 2012, sewerage overflows reaching waterways have breached environmental protection authority licences.

Figures obtained by the NSW Opposition show sewerage treatment plants were put on ‘‘bypass’’ an average of 153 times a year since the O’Farrell government was elected in March 2011. There were 145 occasions last year and 114 so far in the first six months of this year.

Walt Secord, the NSW Opposition spokesman for Water, received information in response to questions on notice which shows sewerage treatment plants at Bondi, Cronulla, North Head, Wollongong and Malabar are among those that are frequently put on ‘‘bypass’’. Mr Secord said treatment plants at Northhead Manly, Bondi and Malabar were 2 km off shore.

‘‘Many of these plants are coastal ones rather than deep ocean outfalls,’’ Mr Secord said.‘‘The community has a right to know when they are swimming in wastewater.

‘‘The O’Farrell government is claiming that they are spending record levels on Sydney Water infrastructure and recent upgrades to its Warriewood plant, but we are still seeing untreated waste water including untreated sewage going straight on to our beaches.’’

Mr Secord said Warriewood wastewater treatment plant was still unable to cope with wastewater overflow despite a recent $32 million upgrade.

Sydney Water collects and treats more than 1.3 billion litres of wastewater a day from 1.7 million homes and businesses in Sydney, the Illawarra and Blue Mountains.

In 2012-13, 5 per cent of the total volume of partially treated wastewater overflow was discharged into the ocean. The previous year it was 4.1 per cent. In 2010-11 it was 1.1 per cent, 1.4 per cent the previous year and 1.1 per cent in 2008-09.

Leigh Martin, an urban water campaigner for the Total Environment Centre said the pumping out of partially treated sewage during times of overflow during sustained wet weather is happening too frequently.

‘‘Bypasses are meant to be an occasional emergency response. It shouldn’t be something that happens every time we get some wet weather,’’ he said.

‘‘It is a function of the inadequacies of the system. It is a function of amount of stormwater getting into the system.’’

Mr Martin said during periods of wet weather leaks in the system are allowing additional water into the stormwater system which is exceeding the system’s capacity.

‘‘It is a problem because it is resulting in untreated effluent being released into the harbour and the oceans,’’ he said. ‘‘The normal tertiary treatments are not happening.

‘‘A number of harbour beaches become unsafe for swimming during periods of wet weather’’.

Stuart Khan, a senior lecturer in environmental engineering at the University of NSW said partially treated wastewater has not been through biological treatment process and presents ‘‘an increased health risk’’.

He said he anticipated an increase in extreme weather events would result in sewage treatment being put on ‘‘bypass’’ more frequently in the future.

‘‘You are bypassing pathogen removal, the removal of viruses and bacteria and the removal of organic material and nutrients,’’ he said.

‘‘Whether it is enough to have a health impact depends on the volume.’’

Mr Khan said whether the wastewater presented a signficant health risk could only be determined by knowing the concentration of pathogenic material in the water.

A spokeswoman for Sydney Water said other Australian states and most developed countries use a similar approach to Sydney Water. She said Sydney Water’s treatment plants are built to cope with three to four times the average dry weather flow.

‘‘This means they treat all normal dry weather flows, as well as a large proportion of the wet weather flow,’’ she said.

The spokeswoman said Sydney Water monitored water quality to ensure no lasting environmental damage was being caused.

A spokeswoman for the EPA said it places conditions on discharges through environment protection licences.

‘‘Sydney Water, as part of its licence requirements, must restrict the amount of various pollutants discharged to waters, monitor discharges to waters, report on their discharges to waters, ensure their pollution control equipment operates properly and efficiently and undertake a pollution reduction program to manage, investigate or address the discharges to waters,’’ she said.

‘‘In cases of specific wet weather overflow incidents reported within the licence requirements, the EPA assesses the threat of pollution to receiving waters. Sydney Water may be required to conduct water testing and report the results to the EPA for further assessment.’’

Number of Bypass events where partially untreated sewerage is released from Sydney Water treatment plants:

2013 - to date: 114
2012 - 145
2011 - 132
2010 - 131
2009 - 86
2007 - 156
2006 - 70