why does new zealand have an absence of ground-based predators? has this laways been the case?

having just returned from a month in NZ and asked the same question I feel well equipped to answer!

NZ broke away from gondwanaland (https://en.wikipedia.org/wiki/Gondwana) before marsupials and mammals evolved. it was sufficiently far away to stop land based animals swimming there (until Maoris and then white settlers arrived less than 500 years ago). birds with no predators evolved to be heavier and thus became flightless and then in turn became ground based predators. they in turn were then almost eradicated by man and the animals that they brought with them. the same is true for much of the indigenous flora which in many cases was on the brink of extinction until conservation projects started.