More of a FYI to potentially save someone all the money, stress and time I wasted on Electrical automotive technicians/ RACQ / parts changing (New Starter battery / Ignition starter switch) and trying to locate the issue.

Dropped my car key but didn't think anything of it (factory FOB), went to start vehicle. No cranking and had the flash code of the immobilizer (3 red - pause - 3 red) in the dash also tried the same for my spare key (which I later found had no transponder in it 2 weeks later
RACQ inspected and couldn't figure out so advised to contact automotive electrician.
$400 later and all I have is a code P1624 and a replace battery advice as was showing a dead cell, replaced battery nothing same symptom. Contacted Locksmith and they advised it is very unlikely 1% that the key would be having issues as the transponders rarely fail. Therefore, I ordered the ignition starter switch as easy and relatively quick to change. completed and still no change. I thought hmm, after dropping keys it's strange that this issue happened.
Decided to hit up the automotive locksmith who scanned my key and found it was still transmitting except a discrepancy (error in a line of coding which was strange)
The locksmith nicely enough programmed and cut a new key and manually modified the code correctly (rather than clone current key in case of replicated issue) and I went home praying to the gods and the car started with no issues with the new key. Glad I got this sorted as was not willing to continue changing **** or replacing complete security sets for a 15year old car.

Just thought I would share as I could not find **** on the internet regarding this issue which made it that much harder to at least guid in the right direction.