THE QUIET EARTH, d. Geoff Murphy (1983)
When suicidal scientist Zack wakes up and finds the world utterly deserted, he soon resorts to exactly the sort of thing we'd all do if we believed we were the last human beings on earth: walking down the high street in torrential rain, wearing a policeman's hat and playing a saxophone, for instance, or cross dressing and giving a rousing speech to an audience of celebrity cardboard cut outs, that sort of thing.
Solitude can do strange things to the lonely mind, and so does absolute freedom.
What happened is only hinted at, but the reason that Zack (and some others, as it turns out) survived is cleverly done and almost plausible. An intelligent, understated film with some mind-bending bits and an uncertain ending, The Quiet Earth is essential viewing, if only as research for the imminent apocalypse.