Thanks for the input guys.
Old and new: It's a 62cmX57cm frame. Yeah, it's definitely no Miyata or Nishiki, as I know them very well. No to Kuwahara too.
I was thinking something like a Panasonic, but like you said, this is pure speculation/guesswork.
I'm in Australia, and I picked it up over here, so even though that doesn't really narrow it down, perhaps it suggests it's a locally-branded frame, built in Japan and then sold by one of the myriad of shops/brands back in the day under their own name.
I can't tell for sure if it's been repainted or not, but I don't see any evidence of stickers/decals, so I think I'll say it has.