I used PhotoShop. The map was easy to do, but keeping it viable without messing up town names and such was the hard part.

The fact that the majority of the offices are coastal is simply a population thing... Throughout history people have moved to where fresh water can be found... Hence the fact that people tend to live more around coastal areas than further inland.

Further inland would mean living on bore or tank water and the average person prefers fresh running water.

That in turn means DoCS offices are going to crop up where the people are.