A video of leading Philippines presidential candidate Rodrigo Duterte joking about a murdered Australian rape victim has provoked a storm of protest.

The footage shows Duterte and his supporters making light of the assault by prison inmates in 1989 when he was town mayor.

He said the female missionary was so beautiful that "the mayor should have been first".

Critics say this shows Duterte is "unfit" for the office of president.

Duterte's supporters however have dismissed the comments as a joke.

The candidate is known for his irreverent speeches and boasts of sexual conquests.

One recent poll had him leading over other contenders ahead of the 9 May election.

'Disgusting endorsement'

The rape comments in a YouTube video are the latest in a string of inflammatory statements - including the proposed mass executions of suspected criminals - made by Duterte.

In the video he refers to the missionary who was murdered while ministering in a prison in the city of Davao in the southern Philippines and goes on to make his remark to laughter at a campaign rally.

President Benigno Aquino's spokesman Herminio Coloma said the comments reflected Duterte's "lack of fitness for the presidency" and his "utter lack of respect for women".

A rival presidential candidate, Vice-President Jejomar Binay, called the comments revolting.

"You are a crazy maniac who doesn't respect women and doesn't deserve to be president," Binay told Duterte in a statement.

Human Rights Watch (HRW) Deputy Asia Director Phil Robertson said the comments "were a disgusting endorsement of sexual violence" for which Duterte should be utterly condemned.

In 2015, HRW described Duterte as the death squad mayor for his tactics in dealing with violent crime in Davao, with more than 1,000 people being killed.