Abduction and harassment accounted for nine of the strikes, while three were for burglary.
The two "second strikes" were both for robbery and extortion, and both were recorded last year.
Since the legislation was introduced in June 2010, the total number of warnings issued in Northland has risen year-on-year.
Whangarei barrister Aaron Dooney said the three strikes legislation should apply only to sexual offending.
"If you're a sexual predator and you're on your third [strike] then clearly it hasn't acted as a deterrent and perhaps it's time they think about whether they should be in society at all."
Violent offending was "different in nature", he said. "Sometimes violence can be spur of the moment, it can involve less premeditation, it can be a family feud or alcohol can be involved."
Sexual offending was almost always premeditated, especially when it involved children.
"The key should be thrown out for them."
Overall, Mr Dooney didn't believe the legislation acted as a deterrent.
In June last year, Whangarei man Arena Duff, 25, received a warning for his part in a crime spree that targeted Asian people.
He was also sentenced to more than nine years in prison for aggravated robbery, kidnapping and car theft.
A few months earlier, Dargaville man Rapata Tipene received his first strike for kidnapping, wounding and threatening to kill a former partner.
The woman was so afraid of Tipene she jumped from a speeding truck to escape him.
Since the three strikes legislation came into force, more than 3700 first strikes have been slapped on offenders nationwide, and 29 convicted criminals are on their second, or "final" warning. No third strikes have been recorded so far. Last year, 1339 first strikes were issued - the highest number for sexual offences, followed by robbery and extortion. Sixty-one received their first warning for homicide offences. Minister of Justice Judith Collins said it was too early to assess the full impact of the legislation. The rise in the number of strikes since 2010 was largely thanks to "lag time" in the law's implementation, she said.
Garth McVicar of the Sensible Sentencing Trust said the low number of second offences showed the policy was working. Under the old system, recidivist offenders could clock up many offences with few repercussions. "If rehabilitation was going to work, it would have worked under the old legislation."