ACT will not be going into formal coalition with National. This is because ACT does not want to risk losing its independence from National and wants to be able to vote against the many aspects of National policy with which it disagrees. Instead, it will gain one or ministers outside Cabinet and negotiate some policy compromises with the National Party.
The near-absence of formal coalition agreements from New Zealand politics now seems to have become cemented. The 2005-2008 Progressive Party-Labour coalition may have been the last we will see. But why not have a formal coalition agreement? Germany, on whose model New Zealand's form of MMP is based, is still going strong with formal coalition agreements after 60 years. Indeed, the coalition agreement for the 2005 "Grand Coalition" between the main SPD and CDU/CSU parties could be purchased at booksellers in a mass-produced version for a few Euros shortly after its signing. Coalition agreements have also been in place even when the parties are more like-minded, such as between the SPD and Green parties.
Moreover, despite claims by ACT to the contrary, National and ACT are cut from the same cloth. In the last Parliament, National and ACT voted in different ways on just 15 bills of 110 in total - and most of these were on comparatively minor issues (e.g. the New Zealand Sign Language Bill). These differences could easily be set out in a coalition agreement for all to see. (N.B. the figure listed at the above link is higher, but that is because ACT did not bother to vote on a few dozen bills, which is a different matter).
Coalition agreements can take time to draw up - but they offer certainty and stability of what will be carried out. They set out the policy tradeoffs and necessary prerequisites so that there are no surprises later on. But even then, they are hardly exhaustive - look at the National-New Zealand First agreement of 1996 and the fiscal parameters accompanying it. While this agreement has been sneered at for trying to pin down every last detail, it hardly tries to do so. What it does is set down each side's bottom lines and forms guidelines for the operation of the coalition government.
The NZF-National agreement is regarded as a failure, but that doesn't mean every agreement needs to be a failure. ACT and National are more like-minded than NZF and National were, which could surely mean a shorter and succincter agreement. Specifically, it could address what would happen on ACT's bottom lines on crime, the Emissions Trading Scheme and cutting government spending. Without a formal coalition agreement, the tradeoffs would happen anyway, but in a much less open fashion and behind closed doors. And by staying outside a formal coalition, ACT - and United Future and possibly the Maori Party - get to accept the power without taking the responsibility for supporting a National-led government, because at any stage they can shove the "blame" back on to the Nats.
Aha, but drawing up coalition agreements "takes too long". Well, so it might. But why should forming a government be a rushed process and why should voters accept so little written records of the closed-door meetings? The fact that APEC is taking place in a week is of little consequence. I don't see the United States rushing ahead with the inauguration of Barack Obama, just so that he can replace George Bush, a so-called "lame-duck" president, at the summit. What should happen is that both Helen Clark, as caretaker Prime Minister, and John Key, as Prime Minister-elect, travel to the summit, as Winston Churchill did with Clement Attlee to the Potsdam Conference at the end of World War II (this happened because the British election results were not clear - eventually, of course, it resulted in a Churchill-led Conservative defeat).
Hide talks about wanting "policy gains" more than ministerial posts. Well, let's see the horse-trading on paper - in a formal coalition agreement.