I'm an Aussie also born in 91 and I have never heard that expression before. The "she'll be" made me wonder though!
That was my experience too. I hear "she'll be right" fairly often, but I hadn't heard "she'll be apples" before. I couldn't imagine any country but Australia coming up with a phrase like that though.
According to the wiktionary[1] it is "rhyming slang" from: