Union officials have launched an extraordinary attack on the Australian Federal Police, accusing the force of adopting an "unbalanced and aggressive" approach to union activities and executing the Turnbull government's union-busting ambitions.

Sparking a flare-up of simmering tensions this week, a Victorian union safety officer has become the subject of a criminal investigation after he tested the stability of a guard rail during a site visit and it immediately collapsed.

A letter from the AFP, seen by Fairfax Media, details the allegation of property damage against the Construction, Forestry, Mining and Energy Union's Peter Clarke. The union said the case was "bizarre" and added to serious concerns that police were responding to political pressure to become more heavily involved in industrial relations matters.

"This is a bizarre use of AFP resources that ought to be used to deal with the serious criminality that goes on in the community," union secretary Dave Noonan said.