Edit: As Jeremy points out below, the Court of Appeal considered the meaning of unavailable for the first time in R v Darmody  VSCA 41, discussed here. Though not specifically considering the (g) provision, the Court did outline some considerations to be taken into account, describing it as ‘a balancing act’.
Section 65 of the Evidence Act 2008 provides exceptions to the general prohibition on hearsay evidence.
In shorthand, these are:
– Duty (made under a duty to make that representation or to make representations of that kind); or
– Res Gestae (made when or shortly after the asserted fact occurred and in circumstances that make it unlikely that the representation is a fabrication); or
– Reliable (was made in circumstances that make it highly probable that the representation is reliable); or
– Against You (against the interests of the person who made it at the time it was made AND made in circumstances that make it likely that the representation is reliable).
For those who survived their studies through the use of mnemonics, it may be worth pointing out that the summary above spells out the acronym DR GRAY. I admit this is not as elegant as VOID (or accurate – I’m sure some readers will already have noted the differences between traditional common law res gestae and paragraph (b)). I’m fairly sure that the drafters of s 84 deliberately worded the section that way to create the mnemonic.
An essential precondition to any of these avenues of potential admissibility is that the maker not be available (hearsay statements where the maker is available are dealt with at s 66). What constitutes unavailable is exhaustively defined at cl 4 of Part 2 of the Dictionary:
4 Unavailability of persons
(1) For the purposes of this Act, a person is taken not to be available to give evidence about a fact if—
(a) the person is dead; or
(b) the person is, for any reason other than the application of section 16 (Competence and compellability—judges and jurors), not competent to give the evidence about the fact; or
(c) it would be unlawful for the person to give evidence about the fact; or
(d) a provision of this Act prohibits the evidence being given; or
(e) all reasonable steps have been taken, by the party seeking to prove the person is not available, to find the person or to secure his or her attendance, but without success; or
(f) all reasonable steps have been taken, by the party seeking to prove the person is not available, to compel the person to give the evidence, but without success.
(g) the person is mentally or physically unable to give evidence and it is not reasonably practicable to overcome that inability.
(2) In all other cases the person is taken to be available to give evidence about the fact.
Paragraph (g) is unique to the Victorian (not very) Uniform Evidence Act. In 2005 the Joint Discussion Paper recommended the inclusion of a provision like (g) and it was enacted in Victoria. The Commonwealth and NSW did not subsequently adopt it.
In Queensland, s 93B Evidence Act 1977 (Qld) provides an exception where a witness is unavailable because the person is ‘mentally or physically incapable of giving the evidence’. The Criminal Justice Act 2003 (UK) also has similar provisions.
Since (g) hasn’t been taken up elsewhere, there’s no body of UEA law on the scope of its operation. Is a medical certificate for the day of the contest sufficient to render a person, ‘mentally or physically unable to give evidence,’? That seems demonstrably unfair to the other party. At the other end of the scale, where the witness lies in a coma with uncertain prospects of recovery, para (g) seems more likely to be satisfied (though unnecessary, since (f) would also seem to apply).
Evidence which is admissible is still open to be rejected in the exercise of one of the discretions. The reason why the witness is unavailable is only one part – sometimes an insignificant part – of whether the evidence will be admitted.