According to their website, https://www.opportunity.org.nz/free_public_transport
For starters, moving to universal free fares is a 10% funding increase, not a 100% one. We already fund around 90% of the cost of public transport from taxes and rates; fares only cover about 10% of operating costs (or roughly $300 million a year on a $3–$4 billion system). Fully free public transport is about getting the most out of the money we’ve already invested into the network.
Avoid success at all costs - Simon Peyton Jones