WA’s Water Corporation has issued warnings about an offshore wastewater outlet.

The authority says people should not swim, dive, kayak or fish within one kilometre of the outlet off the coast of Cape Peron in Rockingham.

Tests have revealed elevated microbiology levels in the area, and more than 150 dead mullet, plus some jellyfish, were found on the foreshore in March.

The Water Corporation says the rising microbiology levels relate to a major upgrade of the Woodman Point Wastewater Treatment Plant.

Part of the $158 million upgrade has effected the efficiency of the physical and chemical processes that are used make wastewater lighter and lower in density, so it mixes with seawater to minimise the environmental impact.

The project will not be complete until October 2019, but the authorities say the specific works that are impacting water quality are due to be completed before May.

More tests are planned to determine when the water will become safe again.