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Mr Garrett is ACT law and order spokesman and has campaigned on tougher punishment for criminals.
He entered Parliament in 2008 and was previously a lawyer and legal advisor to the Sensible Sentencing Trust.
He owned a legal practice in Nuku'alofa, Tonga from 1999 to 2003.
His assault conviction was in Tonga in 2002, Campbell Live reported tonight.
The conviction allegedly followed a brawl with a Tongan doctor over the doctor's ex-wife, the show reported.
The doctor was also charged with assault over the incident and paid a $100 fine.
Mr Garrett was ordered to pay a fine or serve three days in prison -- he paid the $10 fine.
He told Campbell Live he was appealing the conviction.
In a statement issued after the show Mr Garrett said in 2002 he was attacked outside a bar by the head of psychiatry at Tonga Hospital, Mapa Puloka.
"Dr Puloka hit me once from behind, breaking my jaw in two places," Mr Garrett said.
He treated in Middlemore Hospital and subsequently laid a complaint with Tongan police.
"(Dr Puloka) then made a complaint about me and claimed that, after he hit me, I broke loose from a bouncer and hit him in the eye, causing damage."
Mr Garrett said that was not true, and NZPA was unable reach Dr Puloka at his home in Tonga.
ACT leader Rodney Hide said he knew about Mr Garrett's conviction before he stood at the 2008 election.
"He explained the circumstances, they seemed perfectly reasonable to me. David Garrett is a person that's had rough background, he worked on the oil rigs for 10 years.
"Anyone can turn their life around," he told Campbell Live.
Mr Hide said he had done things in the past he was not proud of.
"What I judge David Garrett on is his performance as a Parliamentarian, he's been outstanding."
Mr Garrett was in trouble last year for making lewd comments to a woman staff member in Parliament.
Mr Hide at the time told Mr Garrett behaviour that was ok as an oil worker or private citizen was not suitable for a Parliamentarian.
"I actually believe David Garrett that he never hit this guy...a Parliament is a house of representatives, it's not a house of saints."
Mr Garrett lives near Helensville, near Auckland, with his wife and two children.
The three strikes legislation ensures repeat offenders receive maximum sentences with no parole after being convicted with certain offences.