The GPO was the location for Main Roads Departments eg when it says 25 km to Ingham, they mean to the main Post Office. Well it was back when I worked for Qld Main Roads in the 1980s as a Surveyor. I forget the designated names for post offices, I think GPO was the main one in each town and PO was other branches ?

Of course when it says 25km to X and X has no PO I can’t remember what they used ?

That might be of zero consequence to the ABS or this map.