Why are some members of Parliament called honourable?

A member of parliament who is or has been a minister, the President of the Senate or Speaker of the House of Representatives may use the title 'Honourable'. This title may be used for the rest of their life and reflects the honour and responsibility of their previous role.

The President of the Senate sits in a large red chair. In front of her, the Clerks sit at a table with books on it.

The President of the Senate.

David Foote/DPS Auspic