Police have issued a warning to gym-goers not to leave valuables unattended.
It comes as detectives from the Maribyrnong Crime Investigation Unit arrested a group of teenagers at a Braybrook gym following alleged thefts last week.
A Footscray woman, 18, has since been charged with armed robbery in relation to an alleged incident in Footscray on April 5.
Further charges are expected in relation to the alleged gym thefts, police said.
A Brookfield girl, 13, has been charged with two counts of burglary and theft, theft of motor vehicle and bail offences.
She will appear at a children’s court at a later date.
Detective Sergeant Mark Anderson said there had been a spate of thefts at gyms across Melbourne’s north-west.
“We are continuing to see small groups of two or three youths target fitness centres across Melbourne to steal valuables,” he said.
“This offending has led to serious high-harm offences being committed, which have a significant impact on the community’s safety and feelings of safety.
“We are continuing to work with fitness centres to address security issues and implement crime prevention measures such as lockable storage devices and CCTV.”
Police are advising gym-goers not to leave bags unattended, use secure lockers and not to allow anyone else to use their access pass.
Patrons who see suspicious behaviour should alert a staff member or call triple-0 if immediate police attendance is required.