he things you said about the flooding were all correct but it wasn’t the reason for the move. My point was that 4IP would not have been allowed to move their transmitter permanently to Brisbane with a 2kw power limit, at least not to St Helena Island. The ABA required a certain signal field strength level (that I can’t recall) in the city of licence. The move to Moreton Bay only came because the ABA allowed 5kw directional power so then 4IP was able to demonstrate that it could provide that field strength. The directional pattern that produced that signal level in Ipswich also included strong lobes to the north and south along the coast plus quite deep nulls to the SW and NW (hence the Beaudesert relay).
The same thing happened with 2MW - it had to demonstrate it could provide the required signal level in its city of licence as did 4NA - with both stations actually preferring to move closer to their respective coasts (according to media reports at the time) but not able to due to the signal level restriction.
Please don’t hold me to exact details as it was a long time ago 
Edit:
Found a previous post from @4TAB-1008kHz