A garda who went on sick leave due to alleged bullying and racial abuse he received from colleagues has won a High Court challenge against a decision by the Garda Commissioner to reclassify his illness.

In his judgement, Mr Justice Paul Coffey said he was satisfied to quash a decision deeming that Garda Deming Gao was suffering from an ordinary illness and not from an injury on duty.

Garda Gao, a Chinese native who is a naturalised Irish citizen, has been a Garda since 2008.

He has been stationed at Dun Laoghaire and Shankill Garda Stations, in Co Dublin but went out on leave in November 2016 following a verbal attack by a Garda colleague.

He has since returned to work.

He had complained he had been bullied by other Gardaí for some time. After complaining about this to his superiors he claimed he was avoided, shunned, isolated and branded a rat by other Gardaí.

Represented by Mark Harty SC the court heard that following the November 2016 incident the Garda was deemed unfit for work due to stress by his doctor, and was assessed by the Garda Occupational Health Service.

His injury was initially classified as "an injury on duty."

In May 2017 he was informed by his employer that his injury status had been changed to "ordinary illness" and backdated to the time he went out sick.

He was told the initial classification was issued in error. No other reason or explanation has been given to Garda Gao for the reclassification, he claimed.

Following the reclassification, the commissioner sought the repayment of monies allegedly overpaid to Garda Gao, which he claimed caused him and his family to suffer financially.

Garda Gao claimed the decision was wrong and sought High Court orders against the Commissioner including one quashing the decision to reclassify his injury.

Lawyers for the commissioner opposed the action on grounds including no decision could be made on the classification of the Garda's illness until an investigation into his bullying complaints had been completed.

The Commissioner also argued the decision was the rectification of an error and was a reiteration of the fact Garda Gao's sickness absence classification must await the completion of the bullying investigation.

It was also argued the decisions complained of were fully and properly explained to the Garda.

In his judgment, Mr Justice Coffey, who noted the Garda who verbally abused Garda Gao had been disciplined, quashed the decision to reclassify the illness.

While the Garda Commissioner is entitled to rectify a mistake, no reason or explanation for making the error was given by the employer to Garda Gao.

Such a failure is contrary to basic fairness because if permitted it allows a decision maker to revoke a decision and thereby circumvent judicial scrutiny regardless of the consequences for those affected, the Judge said.

In this case, the Judge added the failure to give reasons for the reclassification of the Garda's injury status had further deprived him of a true understanding of his position and denied him the clarity he is entitled to.

He was satisfied that a failure by the commissioner to describe or explain "the error" amounts to a failure to comply with the constitutional requirement of fair procedures and principals of natural justice.

The matter will return before the court for final orders in two weeks.