Councils had an obligation under the Health Act to promote public health.
"There is a view held by reputable public health authorities and scientific bodies that fluoridation is beneficial and safe."
These included the Ministry of Health, the NZ Dental Association, the World Health Organisation, the NZ Maori Dental Association, the US Academy of Science, the British Medical Association and the Royal Society of NZ.
The ministry evaluated scientific studies on fluoridation and commissioned its own, and it advised local authorities and communities "in accordance with its function to improve, promote, and protect public health" under the Health Act.
The South Taranaki District Council made its decision to fluoridate after a thorough consultation process.
That began with people in Patea and Waverley as part of the council's public submission process when writing its 2011/2012 annual plan.
It wrote to residents on September 26, 2012, to tell them of plans to introduce fluoride to their water supplies and invite them to attend information evenings in the two towns in October.
Two main presentations at the evenings dealt with each side of the fluoride debate.
Mr Laing said that in the month to November 9, 2012, the council got 508 written submissions from members of the communities, local dentists and doctors, various health and Maori organisations, and the Children's Commissioner, as well as four anti-fluoridation groups.
On November 26, 42 people spoke at a hearing held by the council, with council officers later producing a report analysing the submissions and making "a neutral recommendation".
A special meeting of the council on December 10 voted 10-3 in favour of fluoridating the two supplies.
He said it was common ground that the decision was an exercise of a statutory power of decision that fell within the ambit of the Judicature Amendment Act 1972.
He submitted the court should be slow to intervene in the council's decision.
"Parliament has left the decision to fluoridate to the council as a democratically elected local body.
"It is this body that is best placed to weigh competing considerations."
On the plaintiff's claim that fluoridation amounted to mass medication and its use denied human rights under the Bill of Rights Act, Mr Laing said it all depended on how something with a medical purpose was delivered and who delivered it.
The council said three elements were necessary before something could be called a "medical treatment''- it must be provided to a patient, by a health professional, as part of professional treatment.
"If fluoridation has a medical purpose - which is not accepted by the council - this does not mean the council is providing `medical treatment'.
"The council is not a health professional and fluoridated drinking water is not 'administered' to patients.''
He used the analogy of salt manufacturers adding iodine which prevented goitre: they were not providing medical treatment, even though the addition of iodine might have a medical purpose.
"But a doctor prescribing a course of iodine supplements to a patient would be providing medical treatment.''
He said contrary to the plaintiff's submissions, fluoridation did not treat tooth decay, but reduced the incidence and severity of decay.
A commission of inquiry in 1957 found fluoridation was not a medication (it was a food rather than a drug, and water remained materially unchanged with added fluoride) and while scientific knowledge had progressed since then, "the issues material to whether fluoridation is compulsory `mass medication' have not changed''.
He reminded the court the Human Rights Commission looked at fluoridation in 1980 and found it was not a denial of human rights.
Mr Laing traversed various submissions made to the council at its hearing, including one from Dr Robyn Haisman-Welsh, who said the economic argument for fluoridation was strong, especially for communities with lower socio-economic status.
He said in 1999, a group of independent scientists and economists calculated fluoridation would prevent an estimated 74,200 cases of decay over 30 years. The cost of such prevention would be about $4.20 a case, compared to $117.25 a case if there was no fluoridation.
The hearing is expected to end today.