A parent who smacks a child could be charged with assault if the legal defence of "reasonable force" is removed, the Office of the Police Commissioner says.

In a letter sent to a parents' group, the legal section of the office said that if section 59 of the Crimes Act - which gives parents the legal defence of reasonable force - was repealed, parents would not be able to legally smack their children as a form of discipline.

Green MP Sue Bradford's bill to repeal section 59 is before a select committee after passing its first reading in Parliament.

The police letter said it would still be legal for parents to use force to prevent harm to their children - for example, restraining a child from running out on to a busy road.

"However, smacking a child by way of corrective action would be an assault," said the letter, written to the Family Integrity group.

National Party families' spokeswoman Judith Collins said the advice from the police was contrary to what the Prime Minister, Labour and the Greens had been saying to the public about the effect of repealing section 59.

"Now that the police have admitted that smacking a child would be an assault, potential coalition partners Labour and the Greens need to be upfront and tell people the truth - a Labour-Greens coalition would make criminals out of good parents."

The police letter said that - as with all assault investigations - the police would consider the amount of force used and the circumstances before making a decision about whether a prosecution was required in the public interest.

"An aggravating factor in any such decision may be the fact that a child is generally more vulnerable than an adult," the letter said.

Sue Bradford said that police message was what she had been trying to convey at meetings around the country.

"To me it actually proves the sanity of what I am trying to do because the police response is exactly what the people who are supporting the repeal are saying."

Ms Bradford said that if the section was repealed police would make a judgment about whether to prosecute a person in relation to the circumstances - just as they do now with assault cases involving adults.

Ms Collins said National believed the focus should be on stopping child abusers, not on "criminalising good parents trying to bring up their children".