The Australian prime minister has weighed in to the controversy around sexist comments posted online about an image of the Australian rules footballer Tayla Harris, labelling those responsible “cowardly grubs”.

The photo of Harris soaring mid-air following through on a kick 40m out from goal was posted to Channel Seven’s AFL Facebook page, was removed after a number of sexist comments and then reposted by Channel Seven with an apology.

Harris has said the derogatory comments amounted to “sexual abuse” and called people who who made them “animals”.

On Thursday Scott Morrison added his condemnation for the sexist remarks on what he called a “cracker photo”.

“I think they’re grubs,” he told reporters in Melbourne. “I think they’re cowardly grubs, who need to wake up to themselves.”

Morrison said trolls on social media are “nothing new these days sadly” but noted they “tend to target women”, who “tend to be the target for an inordinate share of the abuse that happens online and I think that’s a shameful indictment on the grubs who get on there”.

“Would they say it to our face? No, they haven’t got the guts to do that. They’re cowards. They’re weak.

“They are … acting out some sort of hatred in a way which really just lessens them as people.”

Harris is a professional boxer who plays for Carlton in the Australian Football League Women’s competition.

On Thursday Harris thanked people who “messaged, posted, commented and shared” messages of support, called out trolls and convinced Channel Seven to reinstate the photo.

“Things have changed … if you’re not with us, you will be left behind,” she posted to Twitter.

Talking at Carlton’s home Ikon Park on Wednesday, Harris said she hoped her experience would prove the catalyst for change.

“If I can stand up here and say something about it and start the conversation ... if that helps one person or heaps of people then that’s what I want to do,” Harris said. “I’m fine with people commenting on and critiquing my football, I understand that is the football beast, but it’s the comments that are severely inappropriate, comments that my family will read.”

News Corp reported on Thursday the AFL’s integrity unit would open an investigation and work with Channel Seven’s social media platforms in a bid to identity the perpetrators. Similar punishments to those meted out to fans who are found hurling abuse inside stadiums – such as club membership bans – could be applied.

“I’ve traded some calls [with 7AFL] this morning,” the AFL chief executive, Gillon McLachlan, said. “I don’t think it’s their issue, it’s more a challenge with the platform, social media, because this is not an isolated incident.

“We’re always talking to them, improving, learning, evolving – it’s a challenging space. When we’re posting stuff I know the guys work hard to moderate that and take comments down. It’s a big wide world out there and you can’t do it to all of them.”