Humphries, who died on April 22 this year, aged 89, was an entertainment icon for decades, known best for characters such as Dame Edna Everage and Sir Les Patterson.
King Charles in a pre-prepared video message will lead a number of musicians, comedians, film directors and politicians who will honour Humpheries today.
The Opera House doors will open at 10.15am, and ticketholders are expected to be seated by 10.45. Latecomers will not be accommodated.
People are only allowed to book a maximum of two each.