Enforcing a government building code which bans unions putting slogans on hard hats or displaying the Eureka Stockade flag has been compared to a game of "cat and mouse".

The Australian Building and Construction Commission has published examples of slogans and logos that breach the federal government's 2013 and 2016 building codes for companies contracted for Commonwealth-funded building work.

While employees and contractors have the right to join a union or employer group, they are banned from using material that could suggest membership of their organisation is required to work on a government-funded project.

The Eureka Stockade flag has been given as an example of a symbol that is banned.