Key did not agree that the judiciary were out of touch, and said in the past when sentences had surprised or confused people, the Law Society or other organisations later found they were consistent with other sentences.
Filipo, 18, was charged with the assault of Greg Morgan, 21, and his friends, Hayden Williams, 22, Kelsey Odell, 22, and Olivia Samuels, 22, in central Wellington in October last year.
News of Filipo's dismissal horrified the victims and their families who say he wasn't held accountable for his actions, with his rugby career given priority over the safety of the general public.
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Wellington Rugby said they were "extremely disappointed" with Filipo's actions, but emphasised it was the judicial system - not them - that dealt with the case.
"The recent news story about Wellington-contracted Under 19s player Losi Filipo has created an understandable reaction among our fanbase and the wider public," the organisation said in a statement.
"Wellington Rugby would like to state clearly our organisation does not support violence in any form.
"Wellington Rugby provided generic information to Filipo's lawyer about the effects of a conviction on a professional rugby player, but was not involved in the court proceedings and has not been privy to the full details of the case."
Players' Association boss Rob Nichol said it's not exclusive to rugby where people get an opportunity for discharge from conviction.
"Taken out of a rugby context, it could be a young lawyer. Their ability to finish their qualification and they stuff up in some way. Whether it's similar to this or something else. You can take it to other careers around driving and often you'll find drivers, who've suffered a drink-driving conviction sometimes there is an opportunity for discharge because of the impact it would have on them effectively losing their jobs," he told Newstalk ZB's Larry Williams.
"This is society who have provided this opportunity and we put people called judges in place to exercise their discretion on when it's applied. People are right to say 'Wellington Rugby are going to have to convince me so that I want to stay with them'.
"I tell you what. No one's got more convincing to do more than Losi himself. He's got a lot of work to do to restore some faith and given the opportunity it doesn't matter whether it's in rugby or any other career he wishes to pursue. He's been let off the hook, he's been given an opportunity, let's hope he takes it."
Upper Hutt mayor Wayne Guppy also wrote a character reference for Filipo based on the time he coached him as a member of the First XV at St Patrick's College in Silverstream.
"I don't condone what he did. What I wrote was entirely based on when I worked with him when he was at school."
New Zealand Rugby Chief Executive Steve Tew said it is a matter for the police and the court.
"New Zealand Rugby does not condone Losi's actions and now that he has left school and is in the Wellington Lions environment he is getting the support he needs to make better decisions," Tew said.
"We do not tolerate any behaviour by players that disrespects women or anyone for that matter.
"We believe young men, like Losi, are better off with rugby in their lives. Rugby provides a positive environment that helps equip players to manage the challenges they will face in life during and beyond their playing days.
"We know we don't always get it right which is why we are reviewing how we can improve the way we educate our players. We want our game to reflect the values of our society and to be a game that welcomes everyone."
Welington Rugby chief executive Steve Rodgers said the organisation became aware of the incident late last year.
"Based on the facts made available to us, and the court judgement that deemed a conviction out of proportion to the gravity of the offence, we continued down the path of providing Losi with a support network to assist in his rehabilitation.
"We also began an internal process that included our newly employed personal development manager working with him on a regular basis with the view to achieving a positive long term outcome."
Filipo has completed 150 hours of voluntary work with a youth development group promoting positive behaviour through sport and has undertaken regular counselling since the incident, Rodgers said.
That isn't good enough for the victims - who were walking to meet a sober driver after celebrating Samuels' birthday - and their families.
Greg Morgan's mother, Kim, told the Herald she believed Filipo's discharge without conviction was wrong.
"We're not doing this just to have a moan about the fact how kids got beaten up; there's a bigger picture here.
"Wellington Rugby should not be allowed to put the safety of the general public in jeopardy just for the sporting career of one individual...
"My son's a big, big boy and he went down hard. If it had been a smaller person that was attacked like he was, they could have been dead."
Odell's mother, Karen, said what her daughter and friends had gone through was "horrendous and horrific".
"They were three minutes away from going home safely after a fabulous night out celebrating Olivia's birthday.
"How cruel to be smashed up... to get a phone call at 3am to go down to the hospital to collect your child."
Greg Morgan said the group were walking on Wakefield St when they saw Filipo and two other men on the other side of the road.
The men began yelling out to the group and then next thing they knew, they were running towards them.
"I remember seeing Losi fired up from the get-go," Morgan said.
"We kept repeatedly saying we just wanted to go home, we didn't want to be any part of it
but he was ready to go; his fists were clenched.
"He kept repeating us that he wanted to fight. I was smashed to the ground and then my head was stomped on over and over."
One of the boys screamed he was dead and pushed Filipo off Morgan, who then punched Samuels in the throat and Odell in the chin, before helping his brother with the assault of Williams.
"We didn't know Losi. We'd been out for Olivia's birthday. It was a good night.
"I feel gobsmacked and lost for words ... Losi didn't even get a slap on the hand."
Almost a year on, Morgan still experiences fatigue and migraines from the concussion and is only able to work three days a week.
His building apprenticeship will take a year longer to complete, as he has been able to work so little, and he can no longer play rugby, which had loved to do.
Odell has had to have dental work and has a permanent scar.