Newcastle Trades Hall has written to the MPs asking them not to turn out for the official event and photo opportunity on May 3 at the BHP Billiton-backed Newcastle Coal Infrastructure Group terminal, in a sign of support of unions' concerns.

It is understood Ms Keneally is at this stage expected to attend, but senior union officials are to approach the Government about the issue.

Trades Hall secretary Gary Kennedy said the infrastructure group's five-year non-union agreement had been registered in the "dying days" of the WorkChoices legislation, despite the region's history of "minimal industrial action".

"We're saying to the politicians, show your support by not going," Mr Kennedy said.

Trades Hall also sought last year the support of the International Transport Workers' Federation.

Federal Newcastle MP Sharon Grierson and Federal Minister and Charlton MP Greg Combet said they would not attend the opening because of commitments elsewhere.

Swansea MP Robert Coombs, a former Maritime Union of Australia official, said he would decline any invitation.

Hunter Minister Jodi McKay said she had accepted the invitation before receiving the Trades Hall letter, but would reconsider closer to the date.