Does it say who did the stormwater plan?
I have also assumed that the subsurface pipes are 90mm, I would have recommended 100mm DWV with regard to the pipe capacity and the soil classification.
A 90mm pvc SN2 stormwater pipe has an internal diameter of 86.2 mm which gives a volume of 5.84 litres per metre and a velocity of 1 metre per second gives 350 litres per minute. If the pipes used are 90mm pvc stormwater, the velocity of water from your roof through a single pipe during a 1:20 ARI would exceed 3 metres per second!!! At this velocity, the pipe would be operating under sub atmospheric pressure.
A 100mm DWV pvc pipe has an internal diameter of 104mm which gives a volume of 8.5 litres per metre and a flow rate of 510 litres per minute at a velocity of 1 metre per second. Draining your roof during a 1:20 ARI would require the velocity to exceed 2 metres per second.
AS/NZS 3500.3 126.96.36.199 Design Procedure states.....
"The general method for designing a pipe drain for a stormwater drain shall be as follows:
(In part) ..."the full-pipe velocity in the outlet pipe is recommended not to exceed 1.5 m/s and shall not exceed 2.0 m/s."
Regardless, it is a moot point because there is no way that a water tank's 90mm meshed overflow orifice will discharge at anywhere near the required volume which is nearly 18 litres per second to prevent the tank overtopping!
The builders in house design team I suppose , I don’t know 100%
Their estimating department stuffed up on the levels saying we need an extra 5k worth of fill , dispite myself and our sales person questioning it , a patronising reply from them they were quite sure we needed fill , we have approx 60+ ton to get rid of ! , we have a credit for the fill
The certifier has signed off on the initial storm installation so he will be my first call tomorrow
Thanks again for your help it’s greatly appreciated