A Philippine presidential candidate who joked about raping a murdered Australian missionary has widened his lead over his rivals despite a backlash from diplomats, the influential church and women's groups.

Rodrigo Duterte, the tough-talking mayor of the southern city of Davao, emerged as the ‘clear front runner’ in a survey that research institute Social Weather Stations (SWS) released Monday.

The candidate, who has promised mass killings of suspected criminals, saw his support rise from 27 percent of respondents in March to 33 percent in April, giving him a nine-point lead over second-placed Senator Grace Poe just two weeks before the vote.

The survey was conducted from April 18 to 20, shortly after a video circulated showing Duterte making the off-colour remark about the missionary in campaign hustings.

Duterte had told laughing followers that the woman was so beautiful he wished he had been the first in line to rape her before she was murdered in a jail riot in his city in 1989.

SWS spokesman Leo Laroza said the joke may have dented Duterte's popularity but did not prevent him pulling ahead of his arrivals in the poll of 1,800 voters.

‘Mayor Duterte has been steadily gaining ground. It's a clear lead. The joke could have affected him in such a way that his score could have even been higher had it not been for that news,’ he told AFP.

Duterte's comments drew widespread condemnation including from the Australian and American ambassadors, while women's groups filed a complaint before the human rights commission.

But Duterte was undaunted, telling the diplomats to ‘shut their mouths’ and warning he was prepared to sever ties with Canberra and Washington over the affair.

Another survey of 4,000 voters nationwide taken by research group Pulse Asia before the remarks hit the headlines also put Duterte in the lead with 34 percent ahead of Poe at just 22 percent.

Human rights groups have accused Duterte of leading vigilante death squads that have carried out over a thousand killings in Davao -- allegations he has boasted about and said that if elected, he would kill 100,000 criminals.

Analysts credit his appeal to popular disenchantment against the political elite in a nation where one out of four still lives in poverty.

‘Duterte's rise mirrors the revolt of the periphery,’ sociologist Randy David wrote in the Philippine Daily Inquirer on Sunday.

‘It is difficult to see how, under a Duterte presidency, the country can avoid entering another period of political uncertainty.’