Sydney Water has denied covering up the cause of contaminated drinking water in Sydney's south, saying it has provided test results to the public.

A health warning was issued to residents in parts of southern Sydney in December last year after reports the water had a chemical or petrol taste.

Sydney Water says the contamination occurred when compounds from bitumen were let into a water pipe during routine maintenance.

But the incident has sparked claims the contamination could be linked to Orica's former ChlorAlkali Plant at Botany, with suggestions Sydney Water may have covered up the incident and botched test results.

Sydney Water denies those claims, saying there was never any threat to the public.

"There has been no cover up of any results," Sydney Water said in a statement on Sunday.

"Apart from some compounds which created changes to taste and odour in the water, results show the water met the Australian Drinking Water Guidelines."

The company said the NSW Department of Health had been given samples and agreed there was no risk to residents.

NSW Health said it was advised by Sydney Water that a water pipe had been returned to service after maintenance without being flushed.

The government department "considered that the most likely cause to be disturbance of the lining of the water main".

A sample of the water found chloroform and bromochloro methanes - which are not found in bitumen.

But NSW Health says the presence of those compounds is "not unexpected" because trace amounts are commonly found in drinking water supplies around the country.

"These compounds are examples of trihalomethanes that form when drinking water is disinfected with chlorine," NSW Health Director of Environmental Health Dr Wayne Smith said in a statement.

Despite the government labelling NSW Health the "independent water regulator" the department did not carry out independent testing.

"NSW Health asked Sydney Water to undertake testing to confirm the nature and extent of the contamination," Dr Smith said.

The incident has prompted the NSW Greens to call for the creation of an independent body that would "properly monitor pollution".

Greens MP and environment spokeswoman Cate Faehrmann said residents living around Orica's former ChlorAlkali Plant at Botany were "scared out of their brains" about contamination.

"The community is not trusting companies like Orica and now companies like Sydney Water to undertake their activities safely," she told reporters in Sydney.

"It's really important the community trust is restored."

The NSW opposition has joined the call for tougher action by the state government, demanding more transparency about the cause the pollution in this incident.

"Unfortunately there is a culture of cover-up by the bureaucrats at Sydney Water and the O'Farrell government must step in and order Sydney Water to detail what caused this incident," NSW opposition spokesman for water Walter Secord told the Macquarie Network on Sunday.

But NSW Premier Barry O'Farrell has rubbished opposition claims there may have been a cover-up.

"The opposition is talking through an orifice that I won't mention," he told reporters in Sydney on Sunday.

Mr O'Farrell said he would happily drink from taps in Sydney's south because the health department had ruled there was no evidence the incident was linked to the Orica plant.