Dr Manhattan posted earlier this week about Wallace v Debs & Anor  VSC 355, dealing with an application made by Victorian police on behalf of NSW police to interview a suspect already in custody.
A comment there gave me the idea of discussing the status of various investigators who aren’t from Victoria.
Investigating officials means Victorian investigating officials: R v Frugtniet  2 VR 297. If interstate police come here to interview a suspect, they need to comply with the legislative and evidentiary procedures for admissibility of interviews in their home jurisdiction. (But of course, there will also be local requirements for gaining access to a suspect.)
A West Australian police officer is not an investigating official under these Victorian provisions (R v Weston, FC V, 05/08/1992), and nor is a Queensland police officer (R v Bartlett, FC V, 14/09/1994. Similarly, an Australian Federal Police officer is not an investigating official: R v Gionfriddo  VSCA 152; R v Frugtniet  2 VR 297. National Crime Authority operatives are also not investigating officials: WF1 v National Crime Authority (1993) 44 FCR 524; WF1 v National Crime Authority (1993) 44 FCR 533. That probably means Australian Crime Commission operatives (which replaced the National Crime Authority in 2003) wouldn’t be considered investigating officials too.
When police want to interview a person interstate, they have several options.
If the person is under sentence, they can be transferred interstate under the Transfer of Prisoners Act 1983 (Cth). But that won’t ordinarily apply for investigation purposes, only trials of charges.
Or, the police can cross the border and attempt to interview the suspect, as the NSW police tried in Wallace v Debs & Anor  VSC 355. That’s probably the most practical option (aside from waiting to see if the person eventually returns to the jurisdiction where they’re wanted).