Taking the Losi Filipo case back to court, is being seen as sending a strong, important message about how New Zealand sees violent crime.

Filipo, 18, was last month discharged without conviction for an attack that occurred in central Wellington in October 2015.

He was charged with assaulting Kelsey Odell, 22, her boyfriend Hayden Williams, 22, Greg Morgan, 21, and Olivia Samuels, 22.

The sentence caused an outcry from Filipo's victims and other members of the public, who said Filipo should be convicted.

Police have now lodged an appeal against the decision to discharge the Lions contracted rugby player without conviction.

Labour's police spokesperson Stuart Nash said there are public campaigns and a supposed police emphasis on reducing violence, but this shows they're committed to it.

"Just to see where the law stands. So, if nothing else on a matter of principle."

Mr Nash said it's easy to see why the case caused such outcry.

"The nature of the crime was such that to discharge without conviction just seems manifestly unfair," he said.

Auckland University Associate Law Professor Bill Hodge said the appeal will be centred around the argument the judge didn't take into account the appropriate sentencing principles.

Mr Hodge said he believes the appeal is based on the need to ensure people have confidence in the way the law is applied around the country.

He said the argument often heard in similar cases is around the fact a conviction would prevent him travelling as part of his job as a professional athlete.

Meanwhile, the mother of one of the victims has an idea on how rugby can clean up its image, saying anyone with a tarnished record shouldn't be allowed to play professionally.

Karen Odell, mother of Kelsey Odell, said she's had some really good conversations with Steve Tew and the General Manager of New Zealand Rugby over the past few days.

She said you can't get in the police if you've got a criminal record, so if they raise the bar the respect will be there.