A man who’s spent almost 20 years in jail for the murder of a top federal policeman has been found not guilty at a retrial.

AFP Assistant Commissioner Colin Winchester was gunned down in the driveway of his Canberra home in 1989.

Mr Winchester was shot twice in the head at close range as he was getting out of his car.

Former Treasury official David Eastman was jailed for the crime but has always maintained his innocence.

After a marathon six-month trial – a jury in the ACT Supreme Court has found Mr Eastman not guilty.

The prosecution’s case appeared extremely strong.

The DPP alleged Mr Eastman had the motive, had been recorded admitting to the crime, had been searching for guns in the Canberra Times, and was witnessed at the scene days before the shooting.

After hearing 36,000 pages of evidence, more than 100 witnesses and dozens of statements, the jury found there wasn’t enough evidence to prove Mr Eastman is the killer.

This year’s re-trial has cost taxpayers $6.5 million but the case is believed to have cost more than $30 million over the past 30 years.

Winchester family spokesman John Hinchey says it’s “another day of mourning for the AFP and the Winchester family”.