WATCH: The gym thief who steals from people while they work out just metres away

Not every thief wears a balaclava, as one Sydney gym learned after being robbed by a woman in active wear.

She came in for a group class - and left with $1600.

CCTV footage captured at the Lifecycle Fitness in Waverley shows the woman putting her bag in a pigeon hole as she arrives before starting to rummage through other people's belongings around 9.25am.

Fitness instructor Matt Garry said the class hadn’t even started before she had “pocketed a couple of hundred bucks”.

The thief arrives and begins going through other people's belongings. Source: 7 News

Unlike other thieves, this one worked up a sweat in the 50-minute group session, stopping to exit, wipe off sweat and steal.

Mr Garry said "she was stealing, or preparing the next purse for the following break to come and steal” as she left class.

No-one noticed her unusual behaviour as she went back and forth to the storage unit.

She is seen walking back and rummaging through the area with her towel at 9.45am and again about 20 minutes later.

The woman does her class, using the breaks to rummage through people's belongings. Source: 7 News

The woman then leaves at 10.20am.

Fitness instructor Ryan Clark said the woman used her towel to cover up what she was doing.

Crime Statistics show Sydney has the highest number of thefts at recreational sites such as parks and pools, with 221 reported incidents last financial year.

At least in this case, police have a clear picture of who they're looking for.

"Someone's going to know her and point her out and we'll be able to alert the other gyms in the area,” Mr Clark said.

The woman leaves class to steal. Source: 7 News