The Act MP who designed the proposed three strikes law says National has expanded it to include offences like bestiality in a "Machiavellian" attempt to make it look unworkable.

National has toughened the law by adding 20 crimes like bestiality, incest and acid throwing to the list of "strike offences" that could see a repeat offender sentenced to life imprisonment with a 25-year non-parole period.

But hardline Act MP David Garrett said many of the new offences arguably did not justify a life sentence and were possibly an attempt to undermine three strikes.

"It may be a Machiavellian move by National designed to sink the three strikes provision. Many will say incest, for example, while a deeply unpleasant offence, should not be a reason to send someone to jail for 25 years."

Three strikes has been introduced to Parliament by National as a condition of the Act Party's agreement to support it as Government.

National has made three strikes a provision of its own Sentencing and Parole Reform Bill, which is how it added the extra offences.

National has only agreed to allow three strikes to get to the select committee stage. Privately, senior National MPs have reservations and do not believe it will work.

Mr Garrett made the "Machiavellian" accusation in a speech to Act's conference over the weekend.

He told the conference three strikes was designed for "truly serious" offences like murder and rape, which avoided confusing the public into thinking a shoplifting offence could see someone sent to jail for life.

Justice Minister Simon Power said Act had actually agreed to the expanded list of offences.

Mr Power said the list was designed to fulfil National's own parole policy, which would deny parole to those convicted of a violent offence punishable by five years or more if they had committed a similar offence before.

Mr Power said when three strikes was merged into the bill, "as part of that process Act offered to adopt our list of offences, and we accepted."

The Sentencing and Parole Reform bill will go before the law and order select committee this year.

Mr Garrett also told the Act conference he wanted the party to adopt a policy that would free people from criminal liability if they defended themselves in their own home.

Commending the "rolling-pin gran" who appeared on the front page of the Weekend Herald this month, he said "the current law on the degree of force one can use to defend oneself is quite unclear, and is certainly unsatisfactory".

Sexual violation; murder; attempted murder; manslaughter; wounding/injuring with intent to cause grievous bodily harm; aggravated wounding/injury; aggravated assault; assault on a child; cruelty to a child; using a firearm against a law enforcement officer; committing a crime with a firearm.

Compelling indecent act with animal; incest; acid throwing; robbery; aggravated burglary; kidnapping; sex crimes such as indecent assault and attempted sexual connection with a family member who is under 18; abduction for purposes of marriage or sexual connection.