A team of investigators are no closer to sourcing Havelock North's water contamination.
Further test results from Hastings, Flaxmere and Bridge Pa water supplies came back clear yesterday afternoon - despite suspect results on Saturday.
"The Hastings networks are all clear, which they have been for months," Mr Yule said.
"We still have no idea what has caused this to happen."
It showed how marker results were "notoriously" unreliable or false, he said.
The mayor claimed public safety and credibility were the reasons council had chosen to chlorinate the water despite not being sure it was contaminated.
Council chief executive Ross McLeod said receiving the clear results was "expected, but also a huge relief".
"What [yesterday's] results mean is that the Hastings supply, which also provides water to Flaxmere and Bridge Pa, can continue to be considered safe to drink."
He said council will continue to chlorinate the supply in the short-term as required by the drinking water standards.
In Havelock North, the water continues to be chlorinated and a boil water notice remains in place until the health authorities are confident the issue is limited to campylobacter, which is killed by chlorine.
Council staff also took to the skies in a helicopter yesterday to scour Havelock North for clues on what contaminated the water.
Mr McLeod said council said the bird's-eye view had revealed things not easily seen at ground level and would show up any "out-of-the-ordinary earthworks", or other types of ground features that might have affected the areas around the bores.
"We are taking a number of approaches to tracking down the cause - this is one of them."
Meanwhile the Hawke's Bay District Health Board yesterday said the numbers of people seen by general practice and in hospital with gastro illness continued to decline.
Yet Dr William Rainger, Acting Medical Officer of Health for Hawke's Bay District Board, said people with other underlying medical conditions and older people may have the illness for longer and therefore there could be a recurrence of symptoms.
"All the evidence we are seeing, since the water was chlorinated, is that outbreak is waning and there is no evidence of a second wave," Dr Rainger said.
As of yesterday, the district health board had 153 confirmed notifications of campylobacter and 356 probable making a total of 509.
Hospitalisations to Hawke's Bay Hospital are currently sitting at seven in general wards and there are no patients in Intensive Care. GPs saw about 30 patients on Saturday.
Pensioner Jean Sparksman, who died on August 13, had contracted campylobacter.
A post-mortem examination revealed she also had other significant underlying health issues.
Mr Yule said a visit to Mrs Sparksman's family at the weekend went "really well". "I don't want to talk too much about it because of privacy".
The coroner has launched an investigation into the 89-year-old's death.