Thanks out to Betsy, Natalie and Jay who told me about this movie (independently!) ages ago. I just didn't have the opportunity to watch it until recently, though I did sometimes watch the actual Sabrina, the Teenage Witch TV series while it was still going on.
Sabrina the teenage witch, who'd originated in the Archie comics, ended up becoming a franchise of her own when Melissa Joan Hart got the chance to play the character in a live-action TV series. The TV series itself was fun, and it got a couple of animated spin-offs along with two made-for-TV films. Sabrina, Down Under, released in 1999, is one of those films. For the record, I have zero information whatsoever on the matter, but I suspect that the cast jumped on the chance to make this movie because who wouldn't want to film a carefree film like this in the surf 'n sand of Australia?
The premise is pretty straightforward. Sabrina goes on holiday to Australia, where she meets up with friend and fellow witch Gwen (Tara Strong). It's supposed to be a relaxing break for both of them, but they get pulled into some dramatic shenanigans when they cross paths with Barnaby, who happens to be a merman. Barnaby needs their help because he's suffering from a strange affliction that Sabrina recognises as being caused by industrial pollution. Other merfolk are being affected the same way, and to make things worse, a marine biologist named Dr. Julian Martin starts to suspect that merfolk are involved, and makes it his mission to find and expose Barnaby's merfolk colony.