The risk obviously increases with the length of exposure period, and that applies outside NSW and VIC as well
Not necessarily. That only holds true if you are actually exposed.
So if you are in close proximity to a known positive i.e living with them, then exposure duration is a factor, but meerly being in NSW or Vic for 7 or 8 days makes no material difference, especially if the friends or family you are spending majority of your time time with are all fully vaxed and well.
Chances of catching covid from a fleeting interaction with a random stranger are no greater on day 1 of a trip than day 8.
7 days is a completely arbitrary number, not scientific at all. Plenty of people have developed covid symptoms within ia much shorter time frame with Delta.
Using the flawed 7 day rule the majority of nsw would have had covid instead of only the 0.8% who have.