I believe from the hints in the original Mad Max movie and the intro from the second Mad Max movie that the original Mad Max movie was placed after the decline-of-society-events (later established as a nuclear war over energy resources in 2 and 3). The hints in the original movie seem to be:
- The police station seems worn down and wrecked beyond what one might expect
- Populations seem extremely limited, sparse (As a counter: could be either just Australia or limited production budget)
- The government's ability to respond to crime seemed extremely limited
- Government overall seemed relatively unorganized and make-shift
Yet even so the original Mad Max setting seemed more or less modern civilization (it was a place where people took vacations from their job, and could reasonably expect ice cream shops to be on beaches), yet in Mad Max 2 and 3 society is instead near comical levels of post-apocalyptic with only barbaric traces of modern society remaining.
Further, the intro of Mad Max 2 seems to establish the apocalyptic events happening before Mad Max 1. In the intro narration it's said that (portions removed to shorten the passage):
...two mighty warrior tribes went to war... Their world crumbled. The cities exploded. A whirlwind of looting, a firestorm of fear. Men began to feed on men. On the roads it was a white line nightmare. Only those mobile enough to scavenge, brutal enough to pillage would survive. The gangs took over the highways, ready to wage war for a tank of juice. And in this maelstrom of decay, ordinary men were battered and smashed... men like Max... the warrior Max. In the roar of an engine, he lost everything...
Ignoring the titular character, the original Mad Max and the two sequels seem to be in entirely distinct, separate universes.
Is there an in-universe explanation for this apparent discrepancy?
(Note: I haven't seen Fury Road yet, so perhaps that explains something. If there's answers there, feel free to draw from it for your answer without fear of spoiling)