Just noticed this has been inserted into the T&C of the Sun Pass
Peak Travel Period
- A Peak Travel Period applies between December 20 to December 23 and January 5 to January 9 inclusive. When a travel date is selected within a Peak Period, a Fare Premium will be charged at time of booking confirmation. The Fare Premium is non-refundable and subject to change at any time.
Each pass type seems to have different prices