Police have failed in their attempt to make a Northbridge nightclub close early to curb alcohol-related crime, despite evidence there is a serious assault inside the venue every three weeks.

The Deen nightclub has had its extended trading permit reinstated after a year-long legal battle with the Police Commissioner.

The Aberdeen Street nightclub sought a new five-year permit in May 2012 to allow it to operate between midnight and 2am, the extended hours it had had since 2007.

Police and the executive director of Public Health intervened in the application. However, on March 5 last year, the Director of Liquor Licensing granted the extended permit until March 2018.

Two weeks later, the police applied to have the decision reviewed by the Liquor Commission.

Police argued the permit was not in the public interest because "the level of evidence before the licensing authority explicitly shows that there is currently a serious level of alcohol-related harm occurring in Northbridge with a sizeable level of alcohol-related harm occurring within and outside the premises during the trading hours of the (extended trading permit)".

A report by a senior constable from the liquor enforcement unit said that between June 1, 2010, and May 31, 2012, there had been 70 offences recorded outside the venue and 87 inside.

It found there was one serious assault inside the club every three weeks, with 33 assaults occasioning bodily harm during that period.

The Liquor Commission acknowledged Northbridge experienced high levels of alcohol-related harm but the statistics were "not sufficient to come to the conclusion that the premises is the location of a large number of offences".

It granted the permit with reduced trading hours, forcing the club to close at 1am instead of 2am "to mitigate potential for unacceptable harm and loss of amenity".

But in a recent Supreme Court ruling Justice James Edelman quashed the Liquor Commission's decision, suggesting there was a lack of evidence and lack of rational explanation about the particular reduced trading-hour conditions.