A warning has been issued to the public against swimming or fishing in the Wairoa River due to its mouth being blocked.
A Wairoa District Council spokesman said the river mouth had been blocked by a sandbar since last Tuesday.
The river was at a standstill and "slowly rising," as it had no outlet.
Wairoa District Council environment officer Victor Minter issued the warning to the public not to swim or take any fish from the river yesterday, as a "precautionary measure".
The Hawke's Bay Regional Council was responsible for clearing the sandbar and staff would do so this afternoon if conditions allowed.
"The regional council has been testing and monitoring water quality in the river and although the latest results show that bacterial levels do not yet pose a risk, the potential for further bacterial build-up remains."
The Hawke's Bay District Health Board had also been notified and the regional and district councils would take further samples from the river.
The warning would be lifted if these samples indicated acceptable bacteria levels.
The spokesman said the river becoming blocked was "very rare", with the last known instance three years ago.