Labor leader Bill Shorten has ramped up pressure on the Coalition to take a stronger stand against racism by calling on Prime Minister Scott Morrison to back a parliamentary code on "race ethics" in the wake of frenzied debate over One Nation and election preferences.

Mr Shorten has also tried to pull his own trade union backers into line, repudiating their calls to "put the Liberals last" and instead arguing Pauline Hanson's fringe party should always be placed at the bottom of how-to-vote cards.

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

A parliamentary code of conduct on race and cultural ethics was endorsed by Labor MPs in 2016 after Senator Hanson's incendiary speech about Australia being "swamped by Muslims" — but it was not backed by the Coalition at the time.

In his letter to Mr Morrison, the Opposition Leader said the Christchurch terrorist attack against Muslims and its aftermath — particularly Fraser Anning's comments blaming the massacre on immigration — was a chance for the Coalition to "rethink its decision to not sign the code".