SAND from a Gippsland quarry being used to replenish eroded Portsea beach is being washed out into Port Phillip Bay by high tides.

Locals are aghast at the actions of Department of Sustainability and Environment contractors, with a Sorrento-based mariner claiming a huge pile of sand dumped on the beach ready for spreading was washed away by a high tide last Wednesday week.

"This occurred due to a flood tide and a northerly wind, and higher tides are predicted for next week," he said.

World renowned coastal engineer Gerry Byrne of Sorrento said the erosion had been caused by extra swell generated in Bass Strait or the Southern Ocean and called for detailed mathematical modelling of refraction and diffraction of waves as they travel from outside The Heads to Portsea.

"As a general principle, if we don't know what is causing a problem, we are unlikely to be able to fix it. The government, through DSE, is spending more than $1million on 'fixing' Portsea and will be monitoring it for the next two years. I think they should be doing proper studies and doing them quickly."

Peter Watkinson of DSE said: "We are confident that the works we are doing are best practice."

The government is repairing the pier, replenishing the beach with 10,000tonnes of sand and installing 1000 sandbags to protect eroding dunes.