Allegations a Labor senate candidate was blackmailed with intimate images during a hotly contested Australian Workers Union election have been referred to the Australian Federal Police by the unions watchdog.

Alana Herbert, who is third on Labor's WA upper house ticket behind senators Louise Pratt and Pat Dodson, told investigators from the Registered Organisations Commission (ROC) she was threatened with the release of the images if she did not drop her bid to be the union's WA vice-president.

AWU staff were asked to explain "inappropriate and highly offensive" text messages about WA Labor senate candidate Alana Herbert which included demeaning comments about her and one in which she was likened to a GIF of a semi-naked woman.

Ms Herbert ultimately won an election for the position by a handful of votes.

ROC referred her allegations to the AFP in March to investigate possible breaches of the WA Criminal Code and legislation governing unions.