Hmmm?
They have referred to AS/NZS 3500.3 2003 but there have been several updates since then.
So...you have a detention tank with two 150mm outlets. There is certainly no need for the top outlet, an unmeshed 150mm horizontal outlet with just 200mm of water above the invert (the bottom of the pipe) will discharge, unless restricted in some way, at 16.6 litres per second (Reference is AS/NZS 3500.1 Section 8 Water Storage Tanks figure 8.2. This is 996 litres per minute.
There should be a restricted (smaller) orifice fitted at the bottom discharge, usually 25mm so as to restricts the rate of discharge, it is called the Permissible Site Discharge (PSD), the whole purpose of having a detention tank but maybe the council don't mandate it.
The outlets should also be meshed.
I was generous with my head estimation as I guesstimated from the bottom of the pipe...not the way it is normally done but I was being deliberately generous. A 145mm head is ridiculous. Of interest, it takes x4 the head to roughly double the flow rate.
Your 1:20 ARI looks to be 140mm/h based on a minimum 11.7mm of rain over a 5 minute rain intensity duration.