Opposition Leader Bill Shorten has criticised his trade union backers for encouraging voters to preference Pauline Hanson's "extremist" One Nation party above the Coalition, arguing the right-wing minor party should always be put last.

The Australian Council of Trade Unions has maintained Australians should "put the Liberals last" at the upcoming election, a position that ACTU secretary Sally McManus defended as recently as last week.

Labor leader Bill Shorten repudiated the ACTU's position on preferences, saying One Nation should be put last. Credit:AAP

Anger at One Nation has grown in the wake of revelations by al-Jazeera that its top officials met representatives of the United States gun lobby, discussed weakening Australia's tough anti-firearms laws and explored the possibility of securing tens of millions in donations.

Mr Shorten on Wednesday drew a line in the sand and said it was unconscionable to support Senator Hanson's party ahead of the Coalition, a view he said he had put to union leaders.