US Trade Representative Jamieson Greer has defended a decision by President Donald Trump's administration to apply a 10 per cent tariff on Australia despite a free trade agreement, citing the country's ban on imports of US beef and pork. "We should be running up the score in Australia," Greer told a Senate Finance Committee hearing in reply to a question from Democratic Senator Mark Warner of Virginia. "Despite the agreement, they ban our beef, they ban our pork." Australia has restricted entry of US beef due to mad cow disease concerns for more than two decades, stopping almost all shipments. However, US Democratic Senator Mark Warner came to Australia's defence over the important restrictions, labelling US President Donald Trump's 10 per cent tariff "insulting." Greer grilled Trump over why Australia even copped a tariff in the first place. "Can you explain to me, how with a trade surplus, with this strong relationship Australia got hit with a 10% tariff? Senator Australia has the lowest rate available under the new program… We have a trade surplus with Australia. We have a free trade agreement. Why? They are an incredibly important national security partner. Why were they whacked with a tariff?" With AAP.