A senior Immigration Department manager issued the warning after Border Force public servants, angry about their two-year pay dispute, rallied at Sydney Airport last week to kick off their campaign to have the Turnbull government voted out.

The event was the launch of a union-led campaign to put the wage dispute between Immigration and Border Force public servants and the Coalition government on the election agenda and featured unions members wearing black pants and T-shirts clearly marked with the Community and Public Sector Union's logo.

But according to senior Immigration official Jill Charker, the protesters' clothes "simulated" the controversial border agency's official ensemble and she wants them to cease and desist, pointing out that the public service is supposed to be "apolitical".

"Some of our employees chose to participate in a rally, which actively promoted voting against a political party," Dr Charker wrote to staff.

"These employees chose to wear clothing that simulated the Australian Border Force uniform, which included the words 'Border Protection Officer' on the back and 'Protecting our Community' on the front.

"This type of action has the potential to associate the official activities of the department with the rally in the eyes of the public and undermine the community's perception of the department.

"The Australian Public Service is apolitical.

"As employees of the APS we have a responsibility to maintain the confidence of the community by being professional and impartial."

Dr Charker pointed to her department's guidance for employees undertaking political activities that clearly states workers should not wear anything to such events that might identify them as Commonwealth public servants.

"We hold a privileged position in the community and it is our obligation to maintain the professional image of the department at all times," she wrote.

"I remind all employees of their obligations as public servants."