The Cook Islands pollution watchdog says it's been unable to prosecute anyone for the massive poisoning of the lagoon of the main island.
An Australian scientist, Mark Skinner, has found the bacteria level in Rarotonga's Muri Lagoon is twenty times higher than the limit set by the World Health Organisation.
He says it will take years for the algal bloom the bacteria causes, to disappear.
Tania Temata of the environment service says it has been difficult to identify who is responsible for huge levels of toxins in streams flowing into the lagoon.
"because it's not actually a direct discharge into these waterways. What it is is that some farmers spray or use some pesticide or chemicals for their farming activities but it's not actually discharging it into the waterways."
Tania Temata says she hopes Mr Skinner's briefing for government officials tonight will identify where the pollution is getting into Rarotonga's marine system.