The shocking violence in New Zealand “should give us all cause to reflect on the kind of language we use in public debate”, Mr Shorten wrote in the letter.
“We know that words matter. We know they can hurt, and we know they can lead to hate and violence.”
The code outlines eight principles, including respect for religious and cultural beliefs of “all groups living within Australia” in accordance with the Universal Declaration of Human Rights.
Signatories commit to “speak and write in a manner which provides factual commentary on a foundation of truth” and “recognise and value diversity as an integral part of Australia’s social and economic future”.
The code was signed by the entire Labor caucus in October 2016. Those who have since entered Parliament will add their names next week.
While the Coalition remains divided about where to preference One Nation following the Christchurch attack and revelations the party sought to water down Australia’s gun laws in cahoots with the National Rifle Association, Labor is also grappling with dissent about how-to-vote cards for the upcoming election.
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.
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 the unions.
“My clear view — and I’ve stated it to union leaders — is yes, I know that traditionally you say put the LNP last. But I think One Nation’s performance in recent times and that of [senator] Fraser Anning [mean] I think they have to be put last,” Mr Shorten told reporters.
“I will say this again — I would rather see Labor’s preferences go to the LNP first rather than to the One Nation party.
“And that mightn’t always be in my interests. And maybe One Nation, the political leadership, will seek to punish me for this. Well, I’ll have to put up with that.”
He said any level of tolerance for hate speech and extremism meant “you’re actually part of the problem”.
Coalition MPs continued to argue One Nation should be preferenced above the Greens and Labor. Liberal National MP Keith Pitt, from Senator Hanson’s home state of Queensland, told ABC radio the Greens “should be at the bottom of any ticket that we put up”.
Fergus Hunter is an education and communications reporter for The Sydney Morning Herald and The Age.
Michael Koziol is a political correspondent for The Sydney Morning Herald and The Age.