What is a penalty fare?
Snapper uses a "tag on, tag off" system to work out the best value fare for your journey.
When you tag on your Snapper is charged the minimum one zone for the service you catch. When you tag off, the Snapper system checks if you have travelled more than one zone for that service and charges your Snapper any additional fare that applies.
If you don't tag off, the Snapper system has to assume that you travelled to the end of that journey and your Snapper will be charged a penalty fare.
The penalty fare is set by the bus companies' fare policies. For GO Wellington, Valley Flyer and Airport Flyer the penalty fare is the full cash fare for that journey, from where you tagged on. So if you don't tag off, your fare is calculated to the end of the line, and you don't receive the Snapper discount.
If your Snapper incurs a penalty fare that is greater than your Snapper balance you will not be able to use any travel passes loaded on your Snapper.
See also how do I clear a penalty fare and make my travel pass work again?