Donald Trump's advisers have called him a "genius" after reports he avoided paying tax for 18 years.

"This is a perfectly legal application of the tax code and he would have been fool not to take advantage of it," said former New York mayor Rudy Giuliani, one of Trump's advisers.

"He's a genius at how to take advantage of legal remedies that can help your company survive and grow."

Speaking on ABC, the adviser said Trump has a "fiduciary duty" to maximise profits for his company's investors.

New Jersey governor Chris Christie also called Trump a genius.

"There's no one who has shown more genius in their way to maneuver about the tax code as he rightfully used the laws to do that," he said on Fox News.

Christie - who is the head of Trump's presidential transition team - argued the candidate's records show the tax code is a "mess", something Trump can fix.