Is this a real australian expression?  I’ve not heard it

Exactly!  No it’s not common at all, I’ve never heard it!

More common is ‘fair crack of the whip’, but this too is quite old now, hardly ever hear it anymore.

What do australian people say now then?

“Fair go”

“Easy tiger”, though I think this one comes from either England or America.

I have heard the colloquial saying “Fair shake of the sauce bottle” before, although I thought the more commonly usage was “Fair suck of the sauce bottle!” to tell people they are ‘out of line’ or being unfair in some way.

Kevin Rudd used “Fair shake of the sauce bottle”  THREE TIMES in a speech back in June to try to show he was able to connect with ‘ordinary Australians’  but it came across to many people as being a bit contrived!

“Ohh, Fair Go!” is more frequently used by Australians than “Fair shake of the sauce bottle”.

‘easy tiger’ words came from America I guess, I’ve heard sometime when I saw some American drama.

yes and for me ‘easy tiger’ is a bit different, more like telling someone to relax or calm down, and often as a joke