Does it really? Has it been found that a significant source of infections was illegal immigration? Or that without it there would not have already been internal spreading?
But that's not feasible in most countries.
Austrailia and NZ don't rely on a just-in-time cross border economy like the US and Europe. Goods arrive after being on boats which have been off shore for days or weeks.
Compare with the US-Canada "border closure", where thousands of trucks, and their drivers, have crossed each way every day all year.
Or even trailer drop-off and pick-up for no-chance-of-personal-contact changes.
They just choose not to.
Canadian cross-border truckers arriving back from ??? USA are largely exempt from quarantine requirements upon return.