Earlier in the month, the Greens made it clear they would announce their preferred coalition partner before the election, and released the policy criteria on which the decision would be made. Today, they released their decision - and unsurprisingly, it's to back Labour (provided a good agreement can be reached, of course). The reason why is perfectly captured in their accompanying graphic [PDF]:
DPF will no doubt whine some more about "pre-determined outcomes", but the blunt fact is that Labour is much more supportive of Green policies than National is. And as a party of policy, that is what matters. The Greens are there to implement certain policies. Supporting a government hostile to those policies does not advance that goal.
This does not mean the Greens will refuse to work with a National government. They're an MMP party, and they've already demonstrated that they will work with anyone to advance their policies and improve legislation. But they don't owe anyone their support on confidence and supply, and if National wants that, they will need to become a lot greener.