Hi
In March I would suggest the south-west, around the Margaret River wine region and maybe a little inland towards Pemberton and that area.
There are no formal designated ride routes for roadies here in Western Australia but the following might give you some ideas on routes:
The Western Australian Great WA Bike Ride
Touring Western Australia on Bike
The other thing to keep in mind that in March the south-westerly winds will still be blowing pretty strong. This can make riding south into the wind in the afternoons a pretty hard slog. The Bureau of Meteorology has good
climate data and
wind data available.
Regards
Andrew