Your up on the sunshine coast ?? Cooroy?
Was at a meeting a couple of weeks back where Jimmy Graham (Brisbane chief Plumbing Inspector) Informed us that as of Jan 1st 2002, all Stormwater laid in Brisbane(including domestic) will come under the Brisbane Council Plumbing Inspectors. all pipe sizing to code and correctly laid and bedded.
That will stuff a few blokes up that quote using 90mm.
All advice is given with-out seeing the job, and hence all advice MUST be taken as advice with limited knowledge on the exact situation. NO responsibility can or will be taken. And yes, I am a licensed Plumber and Drainer with my own business in Brisbane Australia