Brian Gaff laid this down on his screen :
Depends whether there are bars of steel inside. On one of my front posts the
concrete split on the top hole showing the rebar and a big chip missing.
That I suspect is why posts are formed with holes already!

All such posts have to have steel reinforcing in them, the would be
very weak and fragile without, even the two foot tall versions once
favoured by councils around council properties for wire fencing. The
usual steel is around 5 to 6mm smooth sided. A metal detector will
easily find it.