Los Angeles: Former Australian prime minister John Howard has re-entered the US gun debate, declaring it is "incontestable" gun-related homicides fell significantly after he introduced strict laws following the Port Arthur massacre.

Speaking on CBS' Sunday Morning TV news program, Mr Howard said he was compelled to act after 35 people were gunned down at the Tasmanian historical site in 1996.

"It is incontestable that gun-related homicides have fallen quite significantly in Australia, incontestable," Mr Howard said.

"I mean, if you had 13 mass shootings before Port Arthur and you had none since, isn't that evidence?