US Democratic presidential candidate Hillary Clinton paid an effective federal tax rate of 34.2 per cent last year and a combined federal, state and local effective tax rate of 43.2 per cent, her campaign said on Friday.

Clinton's running mate, US Senator Tim Kaine of Virginia, along with his wife Anne Holton, paid a federal effective tax rate of 20.3 per cent in 2015, the campaign said.

"Hillary Clinton and Tim Kaine continue to set the standard for financial transparency," Clinton campaign aide Jennifer Palmieri said in a statement.

"In stark contrast, Donald Trump is hiding behind fake excuses and backtracking on his previous promises to release his tax returns."

It is customary for US presidential candidates to make their tax returns public, although they are not required by law to do so.

Clinton's tax returns have been made public, in some form, every year since 1977.

Trump, a New York businessman, and his lawyers have cited an audit by the Internal Revenue Service (IRS) as a reason he has refused to release his returns. Trump has also said his taxes are no one's business and that they reveal little.

The IRS has said Trump can release his tax returns even while under audit.