It will be either the Smartlock Module, the steering lock (not the key barrel or ignition switch itself) or perhaps an aftermarket alarm/immobiliser grafted into the steering column wiring.
Start by taking off the steering column plastic shroud and resoldering any joints where an immobiliser/alarm has been grafted in. At the same time, check the steering lock at the opposite of the key closely and carefully; when they first start coming apart it is hard to detect.
The steering lock issue fails on most Oz Fords; see Australian Ford Forums
My EF Ignition switch has fallen apart - help! - www.fordmods.com
etc to see what to look for.
If you have all the parts, you may be able to repair the lock by putting it back together and re-crimping it into place, but generally once it has failed you will need to buy a second hand replacement and they are not cheap.
If's it's not the lock or wiring, likely you have a sticking relay and or dry joint in the Smartlock Module (it sits in the dash panel above the handbrake handle) . You can try reflowing all the solder on the PC board and replacing all the PC board relays or cleaning the relay contacts but it is generally easier to buy a second-hand replacement on ebay or from a wrecker noting it must be of the same colour with the same number and Fairlane ones can be hard to source. This one Ford 6 93 EB Falcon GLI Smartlock Module in SA | eBay
may not fit; especially if your car is a V8.
If you can get the starter to turn by jumping the starter relay near the coolant recovery bottle (use a piece of wire with two male spade connectors) but it does not start this is a good indicator it's the Smarlock module but also check that the grounding screw for the ECU (which also retains the ECU retainer clamp) in the passenger side kick panel at the bottom of the 'A' pillar is not loose (if so, a bigger self tapper will fix it) and that no immobiliser alarm wiring has been grafted in there (if so resolder the joints after disconnecting the battery). While you are looking at the relay clean the socket contacts with contact cleaner lubricant (not just CRC or WD40) as coolant spilled in there builds up a insulating coating of green corrosion; and try a new relay. But this is unlikely to be the cause given your symptoms. More likely it's the Smartlock Module; steering lock or wiring inside the column shroud.
You'll also need to reprogram your keys after fitting either a new Smartlock Module or steering lock assembly.