The Bentley Post Shop Franchise is located at Shop 44A, the Bentley Centre, 1140 Albany Highway, on the corner of McMillan Street, and provides a full range of services, with post office boxes available as well as passport business. According to the Australia Post web site it is open Monday, Thursday and Friday from 9:00 AM to 5:00 PM, Saturday from 9:00 AM to 12:30 PM, closed Tuesday, Wednesday and Sunday. I find these hours strange for a Post Shop in a major shopping centre, but wasn't able to confirm it.
Access to the Post Shop is from within the shopping centre during shopping centre hours. The Post Office boxes are on the outside of the building and are accessable 24 hours/7 days.