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(13) The absence of medical or other expert evidence showing that the polluted Leaches have been responsible for disease is all the more significant when it is recalled that since the year 1946 Auckland and many other parts of New Zealand have suffered from a severe epidemic of poliomyelitis and suggestions have been made that the -condition of the harbour waters and beaches may have contributed to the incidence of this disease in the Auckland district. Inquiries made by Dr. Thompson at the beginning of the epidemic satisfied him that none of the first fifty cases at least had been caused by infection while bathing, and he also stated that there has been no evidence since then which would suggest that any case has arisen from this cause. (14) Dr. Thompson's evidence on these matters has not been challenged, and this fact and the omission of any reference to them by Dr. MacKenzie and in the petition of the medical practitioners must be regarded as highly significant. Dr. Thompson did not suggest that it is not possible to become infected with poliomyelitis by bathing in sewage-polluted waters, and he explained that the reason for the warning given by the Health Department at the beginning of the epidemic against bathing on the beaches was that this was regarded as a proper precaution to take against a possible- cause of infection. The fact remains, however, that there is no evidence which would indicate that any case of poliomyelitis has arisen from this cause. (15) In our opinion, Dr. MacKenzie and the signatories to the petition failed to support the contention that the discharge of sewage to the waters of the Waitemata Harbour as proposed in the Drainage Board's schemes will be a menace to the health of the community. As regards the contention that the practice of discharging sewage into partially confined waters is out of date, it is sufficient to state that no evidence at all was adduced to support the contention, and the weight of evidence given by expert witnesses is strongly opposed to it. (16) We consider that it is necessary that we should state our opinion about the ■evidence of Dr. MacKenzie and the petition presented by him. It will be readily appreciated that the publication of the adverse views of medical practitioners on the subject of the pollution of harbour waters may well exert a material influence on the minds of the public, who may not understand that medical practitioners, however highly qualified they may be, are not necessarily competent to express an opinion of any value on what is, after all, a technical subject outside the ordinary scope of the •experience and knowledge of a medical practitioner. We were unable to conclude from Dr. MacKenzie's evidence that he had made a sufficient study of the highly technical subjects of sewerage engineering and sewage treatment and purification processes to enable him to qualify to speak as an expert. As for the medical practitioners who signed the petition, there was no evidence whatever that they had made any study at all of these subjects. Without in any way throwing doubts on their good faith or motives, we consider it is our duty, in view of the effect the publication of their opinion may have on the public mind, to state that we think it was unfortunate that they did not attempt to justify their opinion, which, as we have said, was not only not supported by any reliable evidence, but was contrary to the weight of the expert evidence adduced before us. (17) There is another matter about which it seems likely that there is misconception among the members of the public to which reference should be made. By section 34 (3) of the Drainage Act the Drainage Board has the obligation of obtaining from time to time the approval of the Auckland Harbour Board " as to the degree and character of purification of the effluent to be discharged " from the works to be established at Brown's Island. The section also imposes on the Board the further obligation that it " shall not discharge or permit to be discharged therefrom any effluent not conforming to the degree and character of purification for the time being approved by the said Harbour Board." In pursuance of this obligation, certain " standards of purity " have been ..agreed upon between the Drainage Board on the" one part and the Harbour Board and

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