Police allege the accused predator targeted the woman while he was meant to be visiting someone else at Howard Solomon Residential Estate.
Police claimed he led the woman from a common area to her room before indecently assaulting her.
The alarm was raised shortly after and police then tracked the accused predator down.
Police said it is not the first alleged sex attack at the Ferndale facility.
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Worker Armand Lutula Wema was charged in January over separate allegations, accused of assaulting two women.
Management has insisted there has been a major security shake-up since.
In a statement, the CEO assured 9News the facility is safe and that many visitors strike up friendships with other residents and families in the period that they visit their loved one.
An internal review has been launched and the Aged Care watchdog is also investigating.