Parents at Bathurst Public School in central were told of the changes when their children came home to share the news, the Sydney Morning Herald reports.
The original uniform policy read ''navy tailored shorts or navy shorts with school emblem and white polo shirt'' were a suitable option for girls in summer.
But that policy has since been amended to say that girls' uniform as ''summer tunic - blue, white and grey checks with navy tie'', with shorts allowed only for sport.
One of the parents of a girl in kindergarten dismissed her daughter's news, believing she had misheard the announcement.
''It seemed absurd. I really didn't give it much thought,'' Zoe Rodwell said.
Ms Rodwell said her daughter Bella “doesn't identify with gender stereotypes” and likes to wear shorts to school.
School principal Kate White told the Western Advocate that the policy change was pushed forward because some girls had become competitive about brand-name shorts or were wearing them inappropriately.
Ms White also said that if girls wanted to wear shorts their parents only needed to inform the school.