I think it's highly likely we'll all stay at Level 4 until 23:59 on Sunday, that gives a couple of extra days for contact tracing and testing, and sewage testing.
If by 12:00 noon on Sunday 22nd there have been no cases outside Auckland and Coromandel, and no positive sewage tests outside Auckland & Coromandel, then the South Island could go down to Level 2 and the rest of the North Island to Level 3 at 23:39 on Sunday. Auckland & Coromandel stay at L4 for another week.
There would have to be special rules for cross-Cook-Strait traffic.
If there are no nasty surprises, everywhere steps down a level every two weeks thereafter until we're all back at L1 again