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

Key points: Rodrigo Duterte extends lead from 27 to 33 per cent

Rodrigo Duterte extends lead from 27 to 33 per cent The election will be held on May 9

The election will be held on May 9 Has warned he would sever ties with Australia, US over "rape joke" backlash

Rodrigo Duterte, the Mayor of the southern city of Davao, emerged as the "clear frontrunner" in a survey released by research institute Social Weather Stations (SWS).

The candidate, who has promised mass killings of suspected criminals, saw his support rise from 27 per cent of 1,800 respondents in March to 33 per cent in April, giving him a nine-point lead over second-placed Senator Grace Poe, just two weeks before the election.

The survey was conducted from April 18 to 20, shortly after a video circulated showing Mr Duterte making the remark about the missionary.

Mr 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 1989.

SWS spokesman Leo Laroza said the comments may have dented Mr Duterte's popularity but did not prevent him pulling ahead of his rivals.

"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," Mr Laroza said.

Mr 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 he 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 Mr Duterte in the lead with 34 per cent ahead of Ms Poe at just 22 per cent.

Human rights groups have accused Mr Duterte of leading vigilante death squads that have carried out over a thousand killings in Davao — allegations he has boasted about, even saying 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.

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

AFP