eBay's delivery estimate is based on the 2 days, because intrastate is generally 2 days. Australia Post's 2-6 days applies to all regular letters, and that will come up no matter what postcodes you enter- I just tested it with Perth to Canberra, and Aus Post still say 2-6 days, but eBay will use the 6 days (generally) for that.

That being said, eBay's ETAs are still out of whack, and do need revising.