Vincenzo Crupi, a maintenance worker, is charged with the murder of gangland lawyer Joseph Acquaro.
Mr Acquaro, a solicitor who had represented several prominent underworld figures, was shot in front of his East Brunswick gelato shop in 2016.
Crupi was charged with the crime in 2018, but has been on bail since March 2021 amid several delays to the trial.
A Victorian Supreme Court judge in 2018 dismissed a police application to withhold from the defence 600 pages of documents.
The police commissioner argued it could identify a police informant, known as Informer Z, but the judge said the documents could substantially assist Crupi's defence case.
The commissioner appealed to the High Court and three justices on Wednesday overturned the Supreme Court judge's ruling.
The High Court found the judge had failed to show his reasoning in weighing the public interests of fair access to information against the potential threat to an informant.
"It is indisputable that, if the primary judge's orders are given effect to, it is highly likely that Informer Z will be identified, giving rise to a serious risk to his or her safety," Justices Stephen Gageler, James Edelman and Robert Beech-Jones wrote in their decision.
The matter will be referred back to the Victorian Supreme Court for determination.
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