The Council of Trade Unions has expressed shock and alarm at the "very minor" sentence of the Christchurch man convicted of the manslaughter of mother-of-two Christine Clark.

Derek Paul Powell's four-wheel-drive vehicle ran over the 45-year-old at the Port of Lyttelton on December 29, 1999. She died from brain damage two days later.

Yesterday's sentence in the Christchurch High Court of nine months' periodic detention made a mockery of the jury's manslaughter decision, said CTU secretary Paul Goulter. It sent a signal about the right to protest and the safety of picket lines.

He called for police to appeal against the sentence and seek a period of imprisonment for the offender.

Powell, aged 53, found guilty after a 10-day trial and jury deliberations of 12 hours, was also disqualified from driving for two years.

There appeared to be little reaction to the sentence from Powell, Christine Clarke's mother, or the 65 people who crowded into the public gallery.

Powell was told by Justice Graham Panckhurst that his driving had not been the only cause of the tragedy in what had been a highly unusual and difficult situation.

Freedom of movement on the roads was a fundamental right, and the decision by police to allow this movement to be interrupted by picketers was "dangerous."

Justice Panckhurst said the sentence should not be regarded as a measure of the life that had been lost.

Having viewed television footage of incidents at the picket line showing aggressive male picketers, he was surprised police had not taken action earlier on the day Ms Clarke was run over.

Justice Panckhurst told Powell he considered he had been impatient at the picket line and was under stress caused by a real level of violence.

But his driving had fallen below the standards of a reasonable and prudent driver.

It was claimed Powell had not stopped out of fear - "and maybe that was understandable," said Justice Panckhurst.

Powell had considered he was in a situation of peril.

Representing Powell, Philip Hall said the trial had been an ordeal for both families. Powell, a company manager, had pleaded not guilty because he believed he had not been criminally responsible.

Powell believed he had been under some form of attack and moving his vehicle forward had not been unreasonable. He believed the way ahead was clear.

Mr Hall submitted that a charge of manslaughter had been inappropriate, and Powell should have been charged with careless driving causing death.

Powell had been unaware he had driven over anyone. He had been abused and attacked by picketers.

"He should never have been put into that position," Mr Hall said. "The police allowed picketers to conduct their unlawful activity."

Mr Hall described Powell as a decent citizen who had been under huge financial and health burdens because of the trial.

"Prison will not achieve anything useful," Mr Hall said.

Approached for reaction to the judge's criticism of police, district commander John Reilly said police had discretion on operational decisions.

"Officers take into account the need to balance the rights of people to protest with keeping the peace and maintenance of access to public roads," he said.

"In this instance, with protest activity taking place over a sustained period and with only very short periods of interference to the roadway, police chose to maintain a watching brief rather than divert resources from other competing priorities.

"While we regret the tragedy that subsequently unfolded, the operational decision made at the time was appropriate given the information and observations available to the commanding officer."