Preliminary results from a Cook Islands survey of household sewerage systems on the main island of Rarotonga show 90 percent of them aren't working properly.
The director of community health services for the Ministry of Health says a random check was conducted on 50 septic tanks from Muri, Nagatangiia and Titikaveka.
Dr Rangi Fariu says the result wasn't unexpected, especially given the concentration of tourist accommodation in the Muri area.
He says most of the faulty systems use an older style of tank, not the modern, New-Zealand-standard tanks with which all new homes must be fitted.
"Some of them have either broken down or the pipes are not working or it was full and overflowing and also at the outflow side of things the pit is either full or it's not working at all."
Dr Rangi Fariu says the auditor has estimated it'll cost up to 280 million US dollars to fix all of Rarotonga's faulty septic tanks.