If you don't know what a "stat day" is (I didn't), it apparently stands for "statutory holidays" which is like public holidays.
During my brief time in the US, I observed that over there the public holidays tend to be something like "nth Friday/Monday of so-and-so month"; that way you get a long weekend. This is how it's done in some other countries as well.
Unfortunately not in my country (where I'm lucky to have both Saturdays and Sundays off! (I should probably move...) ); if a public holiday falls on a Wednesday, you get the Wednesday off. But on the bright side, there's plenty of holidays here.
The employer can fix up the imbalance -- if there are N bank holidays in the year, give all 80% part time workers 0.8N extra days of holiday allowance above their usual amount (etc), and mandate that they cover the bank holidays with days from their allowance if they would normally work them. (My UK employer does this.)
It’s time and a half and a day in lieu for working a stat where I am. If you work it much outside 8.30-5 penal rates rise, but that part is employer specific rather than legislated.
Some enlightened countries pro-rata stat days, but New Zealand doesn’t.