I've always heard salvia has a tendency to die off after 3-5 years. How long have you had yours?

I agree with Robin that it is some form of S. littae. Look at that recurved lower lip, and the foliage. The flower buds also look like littae. This is no S. elegans, in my humble/ amateur opinion. Also, you took the flower photo in July... which is winter in Oz, correct? S. littae blooms in winter (elegans in fall). To my eye, it doesn't look like S. iodantha... the Luis Saso form can get purple tinted foliage, but I think that its foliage is longer than in your photo. It's night here, so I can't easily go out and check. :) Salvia littae likes shade here in California, so the purple tint may be due to sun?

https://extension.colostate.edu/topic-areas/yard-garden/powdery-mildews-2-902/Powdery mildew. Salvia don't like to be wet on their leaves. Water below leaves. I don't know if you can treat for it, but take away the conditions creating it and maybe the plant can outgrow it. No more water on the leaves if you can help it. Water from below. Humidity is a killer. Keep air circulation around your plants, don't let them get smothered by weeds, etc. Remove the infected parts. Don't over fertilize. Get the kind of fertilizer for encouraging blooms and use the LOWEST amount possible, or less.

Want to plant some purple salivas in my flower bed Which ones are considered the best for NJ area. Plenty of sun where I want to plant these. Thank you