According to The Age in Australia:

“We would have to pay a lot of money,” said Sephery-Rad, noting that most of the government’s estimated one million PCs and the country’s total of six to eight million computers were being run almost exclusively on the Windows platform.

“Secondly, Microsoft software has a lot of backdoors and security weaknesses that are always being patched, so it is not secure. We are also under US sanctions. All this makes us think we need an alternative operating system.”

[…]

“Microsoft is a national security concern. Security in an operating system is an important issue, and when it is on a computer in the government it is of even greater importance,” said the official.