Yes Hong Kong require 21 days, I have an ex-pat friend who was in Scotland in December, he had to holiday for 21 days in Dubai after leaving the UK, just to be allowed ot fly inot HK and is now on day 3 of his 21 HQ in HK.
But those quarantone rules are for international arrivals and those tetsing poistive, not for whole cities of people who dont have the virus and havent been a close contact of nayone who has.
This hasnt proven true in NSW with the outbreaks since Vic reintroduced local transmission to NSW in July. We didnt see wide scale spread into regional areas of NSW. Hell thousands of Sydneysiders were in Byrn and surrounds over Xmas and there wasnt a major outbreak there.
NB cases did infect people outside the NB, but quality tracking saw NSW Health keep those numbers low - no periods of 3 days with over 30 cases anywhere outside the Northern beaches. The other states and territories need to trust they can also control cases when they arise, without puttting millions into lockdowns. You have more tools than clsoing a border.