National MP Nick Smith appears to have broken the law over his public statements on a Nelson family's custody battle, a family law expert says.

Dr Smith has publicly criticised the Family Court's handling of a Nelson couple's fight for custody of their son. Family Court law prevents publication of any details of cases.

The Nelson MP yesterday received a letter from Solicitor-General Terence Arnold raising the possibility of prosecution for contempt of court.

Law Society family law section chairman David Burns today said it appeared Dr Smith had breached the Guardianship Act, which made it clear no one could publish proceedings of the Family Court except with the leave of the court.

"It appears very clear that Nick Smith did not apply for the leave of the Family Court to publish that particular proceeding," Mr Burns told National Radio.

"The issue is 'has Nick Smith, prima facie, breached that provision' and I think the answer is 'yes'."

Politicians criticised the courts from time to time, particularly the outcomes of some cases. But most MPs were cautious over Family Court matters because they did not want to heighten tensions, Mr Burns said.

However, Dr Smith had done such things as issue a press release headed "Smith demands urgent inquiry into parent custody fiasco" in which he said "the case almost amounts to state-sanctioned child stealing".

"Some of his language that he used and the way that he approached it was not of, I would think, a balanced and judgement type of approach," Mr Burns said.

"I think that's where criticism has come, is that he has introduced an element of emotion into this debate which, really, I would have thought an MP of his standing shouldn't do.

"There were quite a number of avenues that he could have gone down without going through a public process of issuing a press release, which I think would have been much more appropriate."

Mr Arnold's letter to Dr Smith raised three issues of potential contempt and concluded: "As indicated, I'm giving serious consideration to commencing a prosecution against you for contempt of court".

Dr Smith said the letter had come "like a bolt out of the blue" and was worrying.

"I have consistently acted in good faith to try and help a desperate family who, I believe, have suffered an injustice," he said in a statement.

"I have been very careful not to reveal the names of the persons involved or to criticise any particular judge.

"My objective has been, and remains, to get an independent inquiry so that the processes of the Family Court and the law might be improved."

Dr Smith said the only similar case in New Zealand political history was that of former National MP and current Auckland mayor John Banks.

Mr Banks was prosecuted by the then solicitor general for contempt of court for his 1986 comments about sex offender Robert Holdem's previous convictions while Holdem was being tried for the murder of six-year-old Louisa Damodran. Holdem was convicted of Louisa's murder.

Mr Banks successfully defended the case but the defence came at a "huge personal cost", Dr Smith said.

"I am taking the solicitor-general's letter very seriously and will be responding fully to it after taking legal advice," he said.

National leader Bill English said Mr Arnold's threat to prosecute Dr Smith was a threat to every MP doing their job of representing their constituents.