The NZ Metal Roof and Wall Cladding Code of Practice is a comprehensive design & installation guide, and a recognised related document for Acceptable Solution E2/AS1 of the NZ Building Code.
In the context of COP Roof drainage, NZBC E1 requires that water discharging into a gutter must have less than a 2% probability of entering a building. This means that those drainage paths inside the building envelope, including valleys and internal gutters, must be designed to accommodate a 50-year rainfall intensity over a short period. Roof Drainage Design discusses how the COP addresses this in more detail.
The above caveat equally applies to the limitations on run length of roof cladding, drainage capacity around penetrations, and discharge of spreaders, which are dealt with in sections XXXX
Gutter catchments outside the envelope, which can discharge freely without entering the building, can be calculated using less rigorous safety factors