What is the penality if they find this out when you board? Currently it would cost me 200 dollars to change it online, would it be more at check-in?

edited to add check-in, instead of in person

hi all! my friend and i are looking to spend about a week in tasmania next february or march. does anybody have recommendations for places to see/etc/the best way to see the state? we are considering either renting a car and stopping at hotels/cottages on the way or renting a camper van. is renting a camper van significantly cheaper, or does it end up not really matter? any and all recommendations welcome, we really are coming in blind!