Katy Perry‘s old feud with Taylor Swift is dunzo — something we already knew, but which was double confirmed by the fact KP crashed her Aussie show … and sang a certain song.

The pop star was one of a handful of famous people on hand Friday for T-Swift’s first Sydney concert — where she was hanging in the VIP tent with Rita Ora and, of course, Travis Kelce … who flew on down to be with his girlfriend.

Katy’s presence was interesting though … on account that she and Tay Tay used to beef.

That’s why her singing “Bad Blood” — which has always been rumored to be about Katy — was fascinating to see … as Taylor belted out the tune onstage, she was being recorded singing along down in the crowd. It’s the ultimate irony … and proof they’re all good now.

Katy even posted a picture with Taylor after the show … and they look very chummy.

If you’re unfamiliar … the whole saga dates back to 2014 or so, when Taylor was first releasing “Bad Blood” — during which she alluded to a rift in a friendship … and most everyone assumed she was referring to Katy, explaining it was a personnel issue.

Taylor talked about it at the time without naming KP … “She did something so horrible. I was like, ‘Oh, we’re just straight-up enemies.’ And it wasn’t even about a guy! It had to do with business. She basically tried to sabotage an entire arena tour.”

TS added … “She tried to hire a bunch of people out from under me. And I’m surprisingly non-confrontational – you would not believe how much I hate conflict. So now I have to avoid her. It’s awkward, and I don’t like it.”

Katy ended up responding with her own diss track, of sorts, and went on to say that their feud was all about backup dancers — namely, Katy hiring some of Taylor’s own staffers.

In the end … they ended up making up by the time Taylor dropped her song/video for ‘You Need to Calm Down’ — in which Katy made a cameo … and was clearly cool with her again.

Now, they’re buddies, it seems, and all that bad blood has turned into water under a bridge.

Speaking of water … there was a lot of it at Taylor’s Sydney show — albeit, well before curtain call. Some fans got caught in a brief downpour as they were filing into the stadium