Prime Minister Scott Morrison has directed the Liberal Party to put Labor ahead of One Nation on how-to-vote cards at the upcoming election in a bid to defuse growing anger about Pauline Hanson's minor party and its ties to the gun lobby.

Mr Morrison said further revelations from an al-Jazeera investigation overnight - in which Senator Hanson seemed to suggest the Port Arthur massacre could have been a government conspiracy - convinced him to draw a line in the sand on preferences.

Prime Minister Scott Morrison has pledged to preference One Nation below Labor on how-to-vote cards. Credit:AAP

"This is a decision which is based on our strong view about the sanctity of Australia's gun laws and to ensure that at no stage that those things should ever be put at risk," Mr Morrison said at a press conference on Thursday morning.

"It is very important - having been the party that introduced those laws - that we ensure that they are forever protected and there can be no compromise when it comes to those issues or any trading on the issue of those gun laws."