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There be a lot of pressure of governments to re-open borders for interstate travel for significant events later in the year such as the Melbourne Cup, etc. We may even see the Boxing Day Test Aus vs NZ for a second year in a row.
WA stated their border would remain closed until Oct, I don't think it will be that late.
QLD for instance had zero new infections Tue/Wed this week.
Personally I think all borders will be open by the end Sep & NZ before the Christmas holidays.
I also think there will be a lot more domestic travel for holidays this Summer as people won't be able to go overseas.

The Federal Government will force the States to co-ordinate in the national interest.