SYDNEY (AFP) - Two Australian election candidates were forced to stand down Wednesday (May 1) as a torrent of offensive social media posts emerged in an increasingly bitter campaign.

Prime Minister Scott Morrison was caught on the back foot after two of his Liberal party candidates in the key state of Victoria were forced to resign within hours of each other following anti-Muslim and anti-gay online rants.

Liberal candidate Jeremy Hearn was sacked after the Herald Sun newspaper uncovered an anti-Islamic tirade spouting conspiracy theories about Muslims taking over the government.

It was soon followed by the resignation of fellow Liberal candidate Peter Killin, who used a conservative blog to encourage Christians to join the party to help stop gay people being elected.

Trade Minister Simon Birmingham defended the party, but admitted the candidate vetting process had failed.

The revelations are a severe embarrassment for a government already trailing in the polls, and provided ammunition for opposition leader Bill Shorten's claim the ruling party is "riddled with right-wing extremists".

In recent days the opposition has also had one candidate resign for sharing anti-Semitic conspiracies, while another apologised for sharing rape "jokes" online.

In an increasing fiery campaign, both parties are learning the value of vetting candidates even in long-shot races where they stand little chance of taking office.

The revelations have also raised questions about the calibre of Australia's politicians and their judgement.

One Nation's Steve Dickson, a former Liberal member in Queensland state, resigned this week after he was filmed groping exotic dancers during a trip to the United States.

A candidate for Clive Palmer's United Australia Party in Victoria state was revealed Wednesday to have threatened a local newspaper with lost advertising dollars if coverage did not improve.