DS248:The risk of being infected by CT in AU is no higher than it is in NZ.
People arriving from AU are more likely to be infected in our MIQ than of having been infected in AU before they left. If infected later in the quarantine period, there is a risk of the infection not being detected until after they have left MIQ.
I think a difficulty is in guaranteeing that someone arriving in Australia from (say) Europe is really covid-free when they leave Aussie's MIQ and get on a plane to NZ. Good luck with that international contact tracing!
And what are we going to do if we open the borders to Aus and they or we get a bad dose of CT? Close the borders again with thousands of people in each other's country and needing to get home?