The first major problem with the accused position is that all the elements of murder is not in dispute - they killed the victim and they intended to kill the victim. The second is that the accused’s version of events is going to be difficult to prove without independent witnesses or other completing evidence; the accused themselves being somewhat of a biased witness.
However, for the sake of argument, let’s assume that either the prosecution accepts the accused version in an agreed statement of facts or the jury reaches that conclusion where it is contested.
The Crimes Act s18(2)(1) provides “No act or omission which was not malicious, or for which the accused had lawful cause or excuse, shall be within this section.”
Legally, one acts maliciously if one intends to cause harm, the reason why one is doing so are irrelevant, so the accused acted with malice notwithstanding their “good” intentions. So that defence is closed.
So, that leaves “lawful cause or excuse”. These are the lawful causes that the law recognises:
- Necessity - irreparable evil
- Necessity - imminent peril
- Mental Impairment
- Lawful force for law enforcement or military operations (not addressed in the article).
Of these, only Necessity - irreparable evil seems arguable.
Unfortunately, necessity is a very tough defence to make out: R v Loughnan  VR 443 at  decided that, among other things the accused needed to prove on the balance of probabilities that the murder was done “in order to avoid certain consequences which would have inflicted irreparable evil”. As described, irreparable evil seems very, very likely; but it isn’t “certain”. Certain means there is absolutely no possibility of a last second rescue even if that would appear to border on the miraculous.
In any event, “the defence has never been accepted in a murder case in any Australian jurisdiction, and it is generally considered there are few if any situations where it could be used as a defence to murder; see R v Howe  AC 417.” That case decided that duress was never a defence to murder and that a person should sacrifice themselves or the hostage rather than committing murder. Analogously, there is no “irreparable evil” that could be avoided by murder.