A spokesman for the government station said Au was leaving to pursue other plans. The broadcaster thanked her for her more than 30 years of service.
Au, who was on leave, will vacate her post on August 18.
The station’s controller for TV, Natalie Chan, has been filling in as an acting assistant director.
RTHK has been hit with a series of resignations since career bureaucrat Patrick Li took charge as director of broadcasting in March.
Another assistant director, Brian Chow, who looked after radio and corporate programming, resigned in April citing family reasons.
A number of producers have also quit in recent months.
But there have also been new additions to the station.
The government earlier appointed former director of administration Kitty Choi as an “adviser to the director of broadcasting”, and senior administrative officer Freda Cheung – as well as two Communications Authority officials – have been brought in to provide “special support” to the station.