The World Game understands Shrewsbury have beaten Portsmouth to land the right-sided utility on loan before the end of the transfer window.
Millar is expected to leave Australia and arrive in the UK next week.
The Shrews, managed by former Hull City and Bolton Wanderers defender Sam Ricketts, currently sit 20th in League One, just above the relegation zone.
They have notched two draws and one defeat in their three league fixtures so far this season.
Millar joined Newcastle from the Central Coast Mariners in 2019 and is off-contract at McDonald Jones Stadium at the end of the 2020-2021 campaign.
The 24-year-old originally hails from Victoria and spent his youth career with Dandenong Thunder and Melbourne City.
After being released by City in 2016, he rebuilt his career in the NPL with South Melbourne. After two years with South he was signed by the Mariners in 2018.
Able to play at right back and right midfield, Millar scored four goals and set up three others in 24 appearances for the Jets last season.
The utility is another player to have left Newcastle in recent weeks, following James Donachie, Dimi Petratos and Abdiel Arroyo in moves to overseas clubs.
Millar holds a British passport, making for an easier switch to the UK.
Both Shrewsbury and Newcastle had not responded to a request for comment by The World Game at the time of writing.
