If the CPI grows by more than 15% between its December 2010 level and its December 2015 level (excluding any changes to the GST rate), Matt pays Andrew $100 (in Dec 2010 prices)I added another $100 to Andrew's, so Matt's on the hook for $200 if RBNZ lets inflation get a bit out of hand.
If CPI grows by less than 15% during this period, Andrew pays Matt $100 (in Dec 2010 prices).
The December 2010 CPI index was 1137. So if the CPI December 2015 is less than 1307, Matt wins his two bets.
Why did I make this bet? Because I bear psychic costs if the inflation target isn't met. It grieves me when quasi-constitutional things are batted aside. And so in the state of the world where I'm depressed, I get paid $100. In the state of the world where I'm happy that RBNZ has followed its mandate, I pay $100. Equalizing utility across potential world states is where it's at, man.