Protesters who had been blockading a Melbourne construction site say they have abandoned their picket line because they have been threatened with legal action.

The group, which included unemployed tradesmen, were blocking access to the Werribee water treatment plant in the city's outer-west.

They were protesting against the employment of Filipino workers on 457 visas.

Protest group spokesman Nick Donohue says they were presented with a letter from Melbourne Water and City West Water telling them they were trespassing and would be charged.

Even though the protest has ended, Mr Donohue says the foreign worker issue will not go away.

"If this happens in the western suburbs, considering the circumstances with the skilled workforce that they've got available, especially when the construction industry has just grinded to a halt, and where locals are paid the rates," he said.

"We know this is wrong, this is very wrong."