The natural suspicion was that the government had ordered the business case removed because it undermined its election promise. And thanks to the OIA, we now know that that is exactly what happened:

Interestingly, Transport Minister Simon Bridges had denied giving any such orders. So either he was outright lying in response to an OIA request, or doesn't know what his own staff are doing. Either way, its not acceptable, and NZTA should not be censoring public information to help with the Minister's election campaign.