Today Haast Pass was closed to all traffic for most of the day due to a rock fall. There is only the one road - if it closes you have two options. Sit and wait till the road re-opens, in this case it took them all day and the road is now only one lane past the slip ... or do a u-turn and head back to Franz then where?
Stranded visitors would have their travel severely delayed, booked hotel rooms will not be used, flights have been missed, some visitors might have to scramble to get to Queenstown or Christchurch for international flights, rental cars might not be returned to the specified location on the rental agreement - thus attracting additional fees.
if you have travel insurance all these costs would be picked up by the insurance policy. No insurance - you pay.
Best to pay a little more upfront when booking your trip to prevent potentially far larger bills later from ruining your holiday.
Safe travels :)