Series 4 (Razor) is Sydney in the 20's, 2 women who run gangs called Kate Leigh who runs a 'Sly Grog' (beer being sold after 5pm) and 'Snow & Kitty Devine who runs knockin' shops and 'snow' which up until then had been available from dentists until they were persuaded to start using novacane.
series 5 (badness) is based on real events in Sydney from 2012
Series 6 (Squizzy) is in Melbourne from 1915-27 about a guy that appears in series 4.
Fat Tony & Co is based on Tony Mokbel from the one with Karl williams.
Underbelly NZ (the Land of the Long Green Cloud) is about events prior to Tale of 2 Cities (series 2).
Underbelly Files is 3 stand alone films
the first was
Tell Them Lucifer was Here about the murder of Victorian police
the second
Infiltration about a fella and his mrs in Griffith (where the weed was grown in series 2) that infiltrate the local mafia
and the third
The Man Who Got Away tells the story of David McMillan who was a British born Australian drug smuggler and the only westerner in history to escape from Klong Prem prison in Bangkok.