Peggy Wong Pik-kiu of the DAB said she had visited the unit in question with a plumber and found the filter installed at the water tap was to blame, as it was too dirty and may have affected the testing.

Five water samples re-tested from the unit found that the nickel level was within the safety limit, she said. "It was irresponsible for the Democratic Party not to re-test the water," she added.

But Democrat Helena Wong Pik-wan rejected her criticism and questioned if the samples taken by the DAB were actually from the same unit. She stressed that the Democratic Party followed a stringent procedure in taking water samples.

Peggy Wong returned uncontested in the Wang Fuk constituency in 2011.

Separately, a Housing Authority committee tasked with reviewing quality control and works supervision of water supplies at public housing held its first meeting yesterday.

The committee's chairman, Cheung Tat-tong, said they had discussed its terms of reference to ensure it would not duplicate the work of other panels set up to investigate the lead-in-water scandal. "We will be looking at different stages of the [water] supply chain in detail, such as pre- and post-contract," Cheung said after the meeting. But he said it was premature to say if the monitoring part went wrong.

"We have been supplied with a lot of documents about the existing mechanism and requirements of the Housing Authority in terms of water supply. We have not yet digested these," he added.

Cheung said the investigation would take from three to five months, with regular meetings being held in its early stages. The next meeting is scheduled for next Thursday.

The government has also set up an independent commission, to be chaired by a judge, to get to the bottom of the widening scandal, with Lower Ngau Tau Kok becoming the fourth estate affected since the contamination emerged last month.

Six tap water samples from the estate were found to contain lead levels exceeding WHO limits. Hundreds of residents will now have voluntary blood tests.