So here's some lines to highlight what is unclear from the photos alone
Under the adventitious roots and the woundwood is a patch of bark concealing the old wound, some of the roots have managed to bridge the wound and graft into the live stem tissues below.
Around the other side is the same story in fact it is even more obvious, the bark that has gradually grown over the wound can be easily lifted off but I didnt want to damage any more of the trees tissues
Now I think this type of injury is caused by water cutting into the bark again and again over many years, then the being stopped...either the watering system is altered, removed or discontinued...my quess for a system of the right era would be an early Travelling sprinkler like the Trailco T50 perhaps. Though the age indicated by the size of growth in the adventitious roots might arguably show that it occured much more recently, in which case popups of impact sprinklers would be the culprits.
Here's a recent water blasted
Alstonia actinophylla 
That photo was taken a year ago and the tree has unfortunately not survived (well half of it anyway) I'll try to get a shot of the dead half and the injury site, if council haven't already removed the tree.