Opposition Leader Bill Shorten has hit out at US presidential hopeful Ted Cruz, saying his claims sexual assaults increased after the 1996–97 gun buyback scheme in Australia were "offensive and incorrect".

On high-profile American radio host Hugh Hewitt's show earlier this month, Senator Cruz said Australia's post-Port Arthur massacre gun legislation meant women were unable to defend themselves from being raped.

"And as you know, Hugh, after Australia did that [gun buyback program], the rate of sexual assaults, the rate of rapes, went up significantly, because women were unable to defend themselves," Senator Cruz told the radio host.

"There's nothing that criminals or terrorists like more than unarmed victims."

Mr Shorten wrote a letter to Senator Cruz in response to the claims, saying tightening gun ownership laws after the massacre was "indisputably the right thing to do".

"Twenty years after the chilling tragedy of Port Arthur, along with many Australians, I regard gun law reform as John Howard's finest achievement in office," Mr Shorten wrote.

"Contrary to your comments, reducing the number and restricting the availability of semi-automatic weapons did not lead to an increase in the rates of violent crime, rape or sexual assault.

"Just as importantly, in the two decades that followed these changes, we have not witnessed a single other mass shooting on Australian soil. Not one.

"Please do not misrepresent the success of Australia's gun reforms in a misguided defence of your own gun laws."

Washington Post rates Cruz's 'whopper' statement

The Washington Post also weighed in on the debate, with its Fact Checker column rating Senator Cruz's comment as a "whopper" of a factual error, the highest rating on its "Pinocchio Test".

The Post analysis found no significant spike or drop after the buyback, but a gradual increase in sexual assault rates over the decade after the scheme came into effect.

The increase was likely affected by a rise in the reporting of sexual assaults and there was not prevalent use of handguns for self-defence before 1996, as Senator Cruz suggested, the newspaper concluded.

The newspaper also concluded that given gun culture in Australia and the US was not comparable, including carrying concealed guns and the ability to carry firearms for self-defence, "politicians should refrain from attributing good or bad changes in Australian crime rates to the buyback program or to the legislative package".

The gun debate has become a hot button issue in the presidential race, with Republican candidates like Senator Cruz and the National Rifle Association attacking President Barack Obama and Democrat hopeful Hillary Clinton's references to Australia's firearm laws.

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