While the arrival of F-16s won't instantly win the war they will instantly turn the tide of the air war, just like Javlin, NLAW, HIMARS did for the ground war. The biggest problem the Ukrainian air force currently have is that they're completely hamstrung buy the fact the Russian air force have planes that can shoot their planes down before they even get chance to fire back* (due to a combination of more advanced radar and missiles), an influx of modern F-16s would completely reverse that position (due to the even more advanced radar and missiles).
With F-16s making sure no Russian planes could get close enough to threaten Ukraine's Su-24/25 aircraft that would drastically increase their operational capability, and potentially even free up the remaining MiG-29 and Su-27 aircraft to conduct air to ground and SEAD missions, these could then be joined by the Australian F/A-18s when they arrive, greatly increasing Ukraine's air to ground capability.
*Technically speaking they could probably get a shot off if they decided not to try an evade the incoming missile but that's not a realistic scenario.
I apologise in advance as this is going to seem overly harsh I know but this is a fantastic example of when I said - people who are not experts but have read some media stories are just saying things which are just plain wrong, as if their opinion is reality when it is far from the case.
In reality, F-16's alone without the rest of the force multipliers NATO/US uses, as shown below, are in for a very tough battle, even if they are given the later F-16C Block 50/52 versions rather than updated but retired export versions which is what is being aimed for (F-16A/B Block 20/30 MLU versions) -
- They have no airborne command & control - no Ukrainian AWACS, only Soviet era ground based GCI currently being used to try to shape the air battle - Russian have AWACS to carry out C&C right at the forward edge of battle for better vision and faster decisions.
- They are out-radar ranged in airborne surveillance - again no Ukrainian AWACS just ground GCI stations without the range to see into Russia deep enough to see how the RuAF is forming up for an A-A fight or spot low flying aircraft - Russia has AWACS and can see deeper into Ukraine to spot the UAF forming up and spot low flying aircraft trying to sneak past.
- They are out-ranged in individual aircraft radars - the Su-35 has a much longer range and more capable PESA radar (est 200nm) and the Su-27 has a longer range but less capable radar (est 100nm+) - the F-16 uses a much smaller planar array radar (est just 80nm).
- They are out-ranged in missiles - Su-35 uses 70nm+ ranged R-77-1 whilst F-16 will probably use the AIM-120C-5 or C-7 with a <70nm range - the official figure is just 35nm but its known to be higher than that.
- They are out-ranged in physical range & flight duration - the Su-27/35 family have far greater fuel range giving them more flight time and can be A-A refuelled - Ukrainian F-16's can't fly as long, can't refuel mid-air and can't fly as far so at some point they will have to return home.
- They can't communicate plans "on the fly" as effectively as they will have the NATO/US secure Link 16 data-link removed and will have to rely on voice comms - Russian Su-27/35 can still use their own S-107-1 data-link unimpeded to pass battle plans around far more quickly.
- They have no EW support from the ground or airborne - Russia has lots of jammer, EW and ESM aircraft in theatre already so the Ukrainian radars will get jammed and their comms jammed too.
I could go on and on with even more but hopefully by now you can appreciate that the F-16's (if/whenever they arrive) are in for a very hard battle against all the odds and all of this goes exactly the same for the "Australian F-18's" you mentioned too, again retired old versions, no matter how well you may currently think they'll do.
Those reasons above are why NATO/US specifically uses multiple aircraft types which support each other in a single "grand plan", leveraging the advantages of each type to nullify the disadvantages of others - F-22 using Stealth to get High Value A-A kills (AWACS, refuellers etc), F-35 using stealth to get A-A/A-G kills taking out ground radars, F-15 to kill enemy fighters, F-16 to kill enemy ground units and all overseen by AWACS to control the battle, able to see deep into enemy territory and pass out new plans instantly across secure data-links whilst refuellers keep planes in the air and EW aircraft blind the enemy etc etc all in aforementioned "grand plan" where everyone is working together, knowing what every piece is doing and covering the deficiencies inherent within each aircraft type. If you take just one single piece of that massive grand plan and throw it into combat against an adversary who has all of that "grand plan" equipment, well thats a suicidal battle as Iraq etc have found out.
Luckily for Ukraine, its fairly obvious by now that Russia's version of the "grand plan" isn't as capable as it should be and nowhere near a capable as a NATO/US one, which is the only reason it wouldn't be a suicide mission to fly F-16's directly against the RuAF rather than just performing a "holding action" of the local airspace which is what the UAF is currently doing, but it will still be a very, very hard fight for the UAF even if they get a whole heap of "indirect" support from NATO/US too.