A US court case could open up access troves of private data, including that of Australians.

The US Justice Department has launched an appeal as part of a five-year legal battle with Microsoft in the Supreme Court.

The request is part of a drug trafficking case in which authorities are seeking access an email held on Microsoft’s servers in Dublin, Ireland.

The court will consider whether American providers of email services should be forced to hand over data within its control when it is stored abroad.

The Australian Privacy Foundation (APF) has signed onto a court brief issued Privacy International in support of Microsoft.

“Notwithstanding their nearly instantaneous accessibility around the world, personal emails stored in a Dublin cloud datacentre are no less physically present in Ireland than are personal letters stored in a Dublin bank safe deposit box,” the brief says.

Microsoft argues that having to hand over the email could would breach Ireland’s data protection laws.

“It puts everyone's emails at risk,” Microsoft's president and chief legal officer Brad Smith wrote in 2017.

Privacy International agrees that if the US Government could seize and review data “hosted on foreign soil”, it would effectively remove those countries’ own privacy and data protection laws.

European Union (EU) officials have warned that the case could undermine its data protection laws, and New Zealand's Privacy Commissioner has filed a brief to the court along similar lines.

New Zealand wants the court to consider “the prerogative of each country — large or small — to apply its own law, including fundamental protections for the rights of its own citizens, to information within its own jurisdiction”.

The US Government’s petition to the court says “hundreds if not thousands of investigations of crimes” are and will be hampered by the Government's inability to obtain electronic evidence.

Traditionally, if courts need evidence for local cases that is held overseas, they use Mutual Legal Assistance Treaties (MLATs), but it is an extremely slow and cumbersome process.

If the US Government wins, it will undermine international law, but if Microsoft wins, law enforcement will be thwarted by the cloud, leading some analysts to conclude that there may be no real winners in the fight.