The most recent quiz from the Judicial College is about unfavourable witnesses and the admissibility of prior inconsistent statements. It came out a couple of months ago, but the JCV has moved a few things around on its website so I looked over it again.
The authors of the quiz give as broad a definition to the word inconsistent at s 38(1)(c) as NSW courts have given to the term unfavourable at s 38(1)(a). (The Act’s Dictionary describes prior inconsistent statements, pretty unhelpfully, as a previous representation that is inconsistent with evidence given by the witness.)
I’d thought a witness who claims not to remember something would not be giving evidence inconsistent with a previous representation they have made. There’s nothing inconsistent about a failure to recall, in the sense that it contradicts or casts doubt on the previous representation. But apparently that’s not required. Stephen Odgers refers to Klewer v Walton  NSWCA 308 as identifying both types of inconsistency potentially engaging s 38(1)(c).
Where the Evidence Act 2008 refers to a witness giving inconsistent evidence, it appears that the evidence given need only be different or not as the party who called them expected to be considered inconsistent.