Four of the unions at the desalination plant at Wonthaggiin southeast Victoria have urged their striking co-workers to return to work today.

The workers on strike were protesting an alleged spying incident involving two managers.

Bill Oliver from the CFMEU said in ABC News Online that about 800 of his workers are still angry that the managers responsible have not been sacked.

"They're looking for the dismissal of the human resource manager and the project director who was involved in bringing these people to the workplace," Oliver said.

Cesar Melham, of the Australian Workers Union (AWU) says his members also want them sacked. But he has recommended they return to their jobs immediately.

The Australian Manufacturing Workers' Union (AMWU) has advised its members to go back to work, but is still working through some unresolved issues.

And union bosses in the Electrical Trades Union (ETU) will recommend their members to go back to work after a meeting tomorrow.

The state secretary of union, Dean Mighell, says he is happy with the way Thiess has handled the issue.

Nev Power, confirmed tonight that the two senior managers had been stood down pending an internal company investigation into the engagement of consultants Australian Security and Investigations.

"Due to the serious nature of this matter, this decisive action is necessary," Mr Power said in a statement.

"The investigation is being conducted by Thiess legal counsel and external lawyers and advisors.

"If anyone has information that may assist with the investigation we urge them to come forward."

The company said it hoped to know the results of its investigation by early next week.