Why is ACC investing in Ruataniwha?

Earlier this year, there were repeated suggestions that ACC might invest in the Ruataniwha irrigation scheme. Like many others, I was curious about whether they were or not, so I asked. And like the others, I received the same response: neither confirm nor deny:

It is understood ACC may invest in the dam, after cornerstone investors Trustpower and Ngai Tahu pulled out, saying the risks were too high and the rewards too low.

However, ACC said its position would be prejudiced if it disclosed whether the information asked for even existed, and said therefore "ACC neither confirms nor denies the existence or non-existence of the information".

Which is pretty much an admission of guilt, in that if they had no plans, they would simply say so. Which invites the question: why the hell is ACC investing in this shitty deal? I managed to get some other bits of data - they are not acting under a formal government direction - so why are they doing this? The market has (quite rightly) turned its nose up at this scheme, viewing it as a risky investment with low rewards; wouldn't investing in it be a violation of ACC's duty to prudently manage public money?