New Zealand is within its rights to forcibly quarantine those suspected of COVID19 who refuse a test on public health grounds. Is it proportional to the threat posed? That could be challenged by judicial review, as it would be in Scotland or the rest of the UK. Calm down.
Will be prevent presentations at the border? No, they'll arrive or they won't. Will it prevent presentation in the community. Obviously it will, and there begins the makings of a judicial review. People really need to have some faith in Western values.