An amusement ride company must pay almost $80,000 after a day at the fair turned to horror for a mum and her two-year-old son.

The pair were on the Mahons Amusements' Crazy Circus ride at the Alexandra Park Fun Fest in January 2015 when the boy became entangled while negotiating his way between a motorised spinner - a padded bag that gently squeezes children as they run through - and a rail.

The boy, who was holding his mother's hand when his shoe became caught in a 10 centimetre bolt at the base of the spinner, was pulled backwards and the spinner rotated five times before staff were able to pull the emergency stop switch.

The boy's leg was broken in three places, and he is still suffering ongoing effects,
needing an orthopaedic insert in his shoe to even the length of his legs.

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He also suffered a 10cm wound to his ankle and struck his head several times, although the latter was not thought to have inflicted lasting damage.

Investigations revealed workers had previously modified the 19-year-old ride, placing the 10cm bolt at its base.

Engineers tasked with regular checks of the ride were not told about the bolt and later said to WorkSafe they would not have certified the ride had they known.

Family-owned Mahons Amusements owner John Mahon knew about the bolt, but didn't think it was a problem.

For that, his company will pay a heavy financial price.

Mahons Amusements pleaded guilty just before a planned trial in June to a WorkSafe health and safety charge of failing to ensure neither action nor inaction of an employee caused harm.

In the Auckland District Court today, Judge Rob Ronayne ordered the company to pay emotional harm reparation of $30,000 to the boy, $5000 to his mother and $1500 to his father, who was not present when the boy was injured.

The company must also pay a fine of $40,000, $650 towards the mother's costs and $130 towards court costs.

Barrister Ian Brookie, for WorkSafe, said there was "simply no excuse" for the engineers to not have been told about the bolt, but agreed there should be a small discount for the company's guilty plea.

Defence counsel Neil Beadle said the company had no previous prosecutions or convictions, and an apology made during restorative justice had been accepted.

A previous offer of reparation was also made, but turned down pending sentencing.

Beadle also tried to argue the alteration was not easy to see, and had not been noticed by the engineers, but was stopped by Judge Ronayne, who warned him not to blame the engineers.

"The [workers] repair I'm sure was well-intentioned. It's the sloppiness of it and the fact that it was not drawn to the attention of the engineers. That is the problem."

The charge admitted by Mahons Amusements had a maximum penalty of a $250,000 fine.

Judge Ronayne started the fine at $70,000, before reductions for the level of culpability and the guilty plea.

He also took aim at those who made "nasty, cruel and uninformed" criticism of the boy's mother online following the incident.

"She was blamed unfairly ... internet trolls have affected her. Ignorant cowards should stay off the internet, but there's nothing I can do about that."

The boy's mother was not immediately available for comment following the sentencing. Mahon declined to comment.