A security guard has been jailed for 4½ years after he led a three-year-old girl from a play area at a western Sydney shopping centre, indecently assaulted her in a stairwell and exposed himself.

The girl, who cannot be named, was playing at DFO Homebush with her sister on December 18, 2016 while her mother shopped for Christmas presents.

Mohammad Hassan Al Bayati, 30, received a report of a lost child and was captured on CCTV walking into the play area and kneeling down as he spoke to the girl.

He waited as she sat down to put her shoes on, then grabbed her hand just after 1.22pm and led her down a secluded corridor and into a set of fire stairs where he exposed his penis and touched her underpants. The pair disappeared for more than 11 minutes before the girl was brought back.