Rex Cameron admitted to breaking into Hargreaves Mall store Body Beyond Belief in September and stealing protein powder worth more than $900.

Later that month the 18-year-old smashed his way into Ruffell Jewellers on Mitchell Street, broke into cabinets and stole almost $7300 of watches and jewellery.

Jewellery worth more than $1300 was also damaged.

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Cameron again broke into Body Beyond Belief in November, but didn't steal anything.

That same month he returned to Ruffell Jewellers and broke a window to gain entry, stealing almost $6000 worth of jewellery.

He pleaded guilty on Tuesday to four counts of burglary and three counts of theft.

In sentencing yesterday, magistrate Megan Aumair described Cameron's actions as "extremely serious offending, almost inexplicable", noting he had committed no prior offences and had strong family support.

Defence lawyer Kim Parker said Cameron's autism would make him vulnerable in custody and asked Ms Aumair to consider a fine, or failing that, a short, sharp term in custody.

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Ms Aumair said a Youth Justice report completed since Cameron's plea identified "concerning issues" and there were no options except custody.

"In my view, a short period of detention is the only option available to the court," she said.

Cameron was sentenced to three months in youth detention.

He had already served nine days.

If not for his guilty plea, he would have faced a term of eight months.

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