SYDNEY - After decades working in law enforcement, New Zealand-born Christopher Paul Cottrell will spend at least a year in prison for trying to cover up his alleged assault of a teenager.

The 44-year-old former State Rail transit officer was sentenced in Sydney's Downing Centre District Court today after being found guilty by a jury of three counts of perverting the course of justice and two counts of persuading a witness to give false testimony.

The charges are linked to Cottrell's alleged assault of a 16-year-old boy in 2004 near Rooty Hill station in Sydney's west.

He was initially convicted of the assault in the local court but was acquitted on appeal to the District Court.

After the alleged assault, Cottrell approached two colleagues with whom he had been working on the day, and said they should lie about how the boy suffered his injuries.

Today, Judge Stephen Norrish said those injuries were not of a serious nature.

"Clearly (Cottrell) was quite concerned that he would be charged and that he might be convicted, therefore jeopardising his employment," the judge said.

Cottrell, who had previously worked as a Corrective Services officer and private investigator, was sentenced to serve at least 12 months in prison, with a maximum term of two years.

His work history means he will be subject to a protection order while in custody and serve his sentence in a "specialist management area".

As a result of the assault allegations, Cottrell's work as a transit officer ended in 2004 and he has since worked as a cleaner, taking a significant cut to his income.

"It is appropriate to say that (Cottrell) has taken a fall from grace of great proportion," Judge Norrish said.

Friends and family cried as the bespectacled, mustachioed Cottrell was led from the court and taken into custody.

- AAP