You can watch it online in 11 parts, thanks to googlevideo. [update: This is no longer available online.]
1000 Misspent Hours gives it good marks, saying
The Quiet Earth is about people rising above their differences— and not in some cliched, touchy-feely, politically correct, it’s-a-small-world-after-all way, either.
The film is most effective in its conveying the barrenness of humanity’s absence.
Far less effective is when the film abandons the lyrical, visual poetry of isolation and introduces two other characters in the second half. Here it merely becomes a trite love triangle – and one that is not entirely satisfactorily dealt with.
DVDTalk also has a review. ListVerse names it one of the best of the 80's.