Paul Justin Noble and Bronson McGarvey appeared in the New Plymouth District Court and pleaded guilty to robbery.

A judge let him off the hook because he had a promising rugby league career, but two months later Paul Justin Noble smashed a teenager's head into a concrete pole.

In November 2015 judge Garry Barkle granted Noble a discharge without conviction for a king hit he dished out without warning at a party in Marama Cres, New Plymouth.

It was a hit that left the victim unconscious for some time.

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At the time Barkle said Noble's blossoming rugby league career and a potential NRL contract in Australia were key factors in the decision to let him off.

But weeks after walking away with a clean record, Noble and a friend were on Ngamotu Rd in New Plymouth when they demanded two teenagers hand over their phones and wallets.

When they refused Noble grabbed one and smashed his head into a concrete pole.

The victim fell to the footpath with a bloody gash to his forehead, but Noble didn't stop the assault.

Instead, he kicked him and punched him as he lay on the ground, before stripping him of his property.

Discharges without convictions have made headlines in New Zealand this year after rising Wellington rugby star Losi Filipo was let off the hook after an assault because a conviction would make obtaining a professional rugby contract "much harder".

That discharge came after Filipo attacked four people, including two women, on a night out in Wellington.

However, after unprecedented public outcry and a police rethink the ruling was appealed and the High Court overturned it.

Filipo was then sentenced to nine months of supervision and his contract with Wellington Rugby was terminated.

On Tuesday Noble was back in the New Plymouth District Court and this time there was no hope of a discharge without conviction.

Noble and his co-defender Bronson McGarvey were both 18 at the time of the offending and pleaded guilty to the robbery.

Judge Chris Sygrove said the attack happened in January and it was against two people who were "younger and smaller" than Noble and McGarvey.

"This was a random, utterly unprovoked, violent robbery on a public street, by two alcohol-fuelled young thugs," Sygrove said.

"You can rest assured you will not be discharged without a conviction this time."

Sygrove said the two victims tried to run from their attackers, but Noble caught one of them.

While he was busy smashing the forehead of this victim, McGarvey had tried to chase the second victim, but failed to catch him.

Instead, McGarvey made his way back to Noble, who was stealing the victim's cellphone, wallet, cap and shoes.

When spoken to by police Noble said he'd had too much to drink and had no memory of the attack, while McGarvey denied the assault altogether and said he found the phone on the ground.

Noble's defence lawyer Patrick Mooney said his client had apologised to his victim at a successful restorative justice conference and had paid an emotional harm reparation of $700.

Judge Sygrove said Noble had only narrowly avoided being sent to jail for the attack. He sentenced him to 10 months of home detention and issued a first strike warning.

Sygrove said McGarvey had a lesser role in the attack, but only because he couldn't catch his victim.

He also issued him with a first strike warning, sentenced him to 12 months of supervision and six months of community detention.