I thought the neatly contoured pattern of rain that emerged after the event points to the possibility that in this event – the Melbourne urban heat island (UHI) might have attracted rain.
Certainly the ranges to the east and north-east of Melbourne did not attract rain from these storm cells like the centre of the urban area did.
This radar image from 6pm is typical, showing the storms erupting in a band – constantly forming and dying.
There is nothing new about the idea of UHI affecting rain – just search – uhi attracts rain – and there is much to read. I remember seeing case histories from Texas over the years. But on a longer term the signals from events a few hours in duration get obscured by other rain patterns.