MTG and MGB are Mt Gambier, MTG is the ICAO code, and MGB is the IATA code
...and often seem to be deliberately different. I don't see why they can't be harmonised now we aren't sending AFTN messages by paper tape & teleprinters. Oh, and the ICAO code is YMTG.
Condolences to the O'Connor people and hope a buyer is found.
Was the effective grounding of their ancient C441's a couple of months ago a factor in this?