Labor's candidate for the seat of Melbourne, Luke Creasey, has withdrawn after social media posts came to light that showed him engaging in inappropriate jokes about rape, lesbians and Catholics.

Key points: Luke Creasey has quit the party after more lewd comments he made on social media surfaced

Luke Creasey has quit the party after more lewd comments he made on social media surfaced Bill Shorten announced less than an hour before Mr Creasey's withdrawal that an investigation was underway

Bill Shorten announced less than an hour before Mr Creasey's withdrawal that an investigation was underway Mr Shorten earlier defended Mr Creasey, saying the matter was different to that of Jessica Whelan, who quit the Liberals after her own social media posts came to light

Mr Creasey, 29, had been forced to apologise earlier this week after it was revealed he had made a rape joke on social media.

Further questionable posts have since been uncovered by the Herald Sun, including jokes Mr Creasey made about lesbians and Catholics.

This meme was shared on Luke Creasey's Facebook account. ( Supplied )

In a statement released by the Labor Party this afternoon, Mr Creasey said the comments did not reflect his views today but that his actions were "an important lesson for young people that your social media footprint will follow you".

"It is clear the right thing for me to do is stand down," the statement said.

"While I made those awful comments many years ago and they in no way reflect the views I hold today, I understand, especially as a member of the LGBTIQ community, that we need to be careful about what we share or like on social media."

Mr Creasey was a food technology teacher at a high school in Melbourne's inner north before he was preselected to represent the Labor Party in the federal election.

The school sent a statement to parents earlier today distancing itself from Mr Creasey, saying the comments were not made while he was employed at the school and that they did not reflect its culture.

"A number of students, staff and teachers have taken offence at the particular Facebook posts, which are clearly inappropriate," the statement said.

"We do not condone the content of his posts."

'Stupid is stupid': Shorten earlier defended candidate

Less than an hour before Mr Creasey quit, Opposition Leader Bill Shorten said the Labor Party was investigating the candidate's online history.

Mr Shorten earlier defended Mr Creasey, arguing the circumstances were different to anti-Muslim posts made by Liberal candidates.

Screenshots emerged this week appearing to show comments made on Facebook by Liberal candidate Jessica Whelan spouting anti-Muslim, anti-immigrant views.

"This Jessica Whelan, she said what she believes now, and it reflects on what she believes now," Mr Shorten said of the then-candidate for the Tasmanian seat of Lyons, who has since resigned.

"What our fellow said, he said it seven years ago, when he was 22, and he's clearly regretted and apologised for it."

Mr Shorten was asked when the "young and dumb defence" stopped applying.

"Stupid is stupid is stupid," he responded.

"I'm not here giving the bloke a pat on the back."

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 1 minute 26 seconds 1 m 26 s Bill Shorten defends Melbourne candidate Luke Creasey for past rape joke.

Prime Minister Scott Morrison accused Labor of double standards.

"You can say or do anything but as long as you say you were sorry later, you can still stand as a Labor candidate," he said.

"I don't accept that excuse and that defence."

Since the closure of nominations, a slew of candidates have had to quit for inappropriate comments.

Because the nominations process is over, their names will remain on the ballot papers handed to voters in those seats.