Hillary Clinton says her husband's affair with Monica Lewinsky when she was a White House intern wasn't an abuse of power because the 22-year-old "was an adult" at the time.

Speaking on CBS Sunday, Hillary also said she believed Bill was right not to resign as President in the wake of the scandal after he was impeached for lying about the affair.

When asked if her husband should have stepped down, Hillary responded: "Absolutely not."

"It wasn't an abuse of power?" CBS correspondent Tony Dokoupil asked.

Hillary responded: "No. No."

The former Secretary of State noted that Lewinsky "was an adult" at the time after the reporter said there were some people who believed a US President couldn't have a consensual relationship with an intern because the power imbalance was too great, according to the Daily Mail.

"But let me ask you this: Where's the investigation of the current incumbent, against whom numerous allegations have been made, and which he dismisses, denies, and ridicules?" Hillary said.

"So, there was an investigation [of Bill], and it, as I believe, came out in the right place."

It comes after Hillary earlier rejected the idea that accusations against her husband were anything like the sexual misconduct allegations against Trump or his new Supreme Court Justice Brett Kavanaugh.

The Clintons are about to embark on a joint speaking tour that could prompt questions about what the Clintons went through, Bill's 1998 impeachment and allegations of sexual harassment.

The House in 1999 voted to impeach Clinton of perjury and obstruction of its investigation into the affair.

The Senate acquitted him.

New York Democrat Senator Kirsten Gillibrand came out last year amid the #MeToo movement saying she believed the former President should have stepped down as a result of his affair.

Bill Clinton said earlier this year that he disagreed with Gillibrand's comments.