The quarantine requirement was sparked by his Melbourne flight's brief stopover in Adelaide,which was experiencing a COVID-19 outbreak at the time.
On Wednesday,the 20-year-old pleaded guilty in Perth Magistrates Court to one count of failing to comply with a direction under the Emergency Management Act after police issued him a move-on notice in Northbridge on December 19.
It follows the young footballer spending more than two weeks behind bars over Christmas before being granted bail on January 6.
He will be sentenced on March 25,with his lawyer ordering pyschological and pre-sentence reports be completed.
Stack,who is understood to have found his time in prison distressing,declined to speak outside court as he entered a waiting car with his manager by his side.
Asked if he would be able to return to Melbourne for training,Stack's manager Paul Peos said the sentencing date would make it difficult.