Here are some of the writers who have contributed to the series:
Underbelly:
* Peter Gawler
* Blake Ayshford
* Fiona Seres
* Roger Monk
Underbelly: A Tale of Two Cities:
* Peter Gawler
* Blake Ayshford
* Fiona Seres
* Roger Monk
Underbelly: The Golden Mile:
* Peter Gawler
* Blake Ayshford
* Fiona Seres
* Roger Monk
Underbelly: Razor:
* Peter Gawler
* Blake Ayshford
* Fiona Seres
* Roger Monk
Underbelly: Squizzy Taylor:
* Peter Gawler
* Blake Ayshford
* Fiona Seres
* Roger Monk
Underbelly: The Fat Man:
* Peter Gawler
* Blake Ayshford
* Fiona Seres
* Roger Monk
Underbelly: Badness:
* Peter Gawler
* Blake Ayshford
* Fiona Seres
* Roger Monk
Underbelly: The Black Hand:
* Peter Gawler
* Blake Ayshford
* Fiona Seres
* Roger Monk
As you can see, many of the writers have worked on multiple seasons of the series. This is common in television, where writers often work on multiple projects simultaneously.
Note: This list is not exhaustive, and there may be other writers who have contributed to the series.