Hi @zingerty,
Welcome to the Bunnings Workshop community, it is fantastic to have you with us.
That style of lock is known as a mortise lock. They are a bit more complex than a standard door lock, as there is a fairly sizable lock body that sits in a pocket in the door. They are common on commercial buildings where different settings are required, and they are very commonly on fire doors.
If you look at the door jamb, usually between the hinges, is there an inspection plate from a fire company?
If so, then this will tell you that it is a fire door, and any alterations to the door and its accompanying hardware would need to be confirmed with that company to ensure it is compliant.
Are you looking to replace the whole mechanism or just the lock?
If you are just looking to change the keys, then you should be able to just replace the lock cylinder. If this were the case, I would suggest you speak with a locksmith, as they will be able to assist you with this.
If you wanted to change everything, you would need to purchase a mortise lock body, a cylinder and handles that match up with the holes in your door.
Let me know what you think. If you have further questions, please don't hesitate to ask.
Jacob
