Today is the last day in office for retiring senators who were elected in October 2004; tomorrow the senators elected last August begin their terms. And as the blanket coverage in this morning’s media will remind you, that means that the nine Greens senators — up from five in the old senate — will hold the balance of power in their own right.

One shouldn’t overstate the importance of this. It doesn’t mean the Greens have “control”: whenever they take a position that other senators disagree with, they’ll be outvoted 67 to nine. And since the Greens in at least some sense sit at one end of the political spectrum, that’s likely to happen more often than it did with the Australian Democrats, who in their heyday also peaked at nine senators.

Even so, it’s a significant achievement. Having won 13.1% of the vote last year and a seat in every state, the Greens will plainly feel entitled to be treated as a serious political force.

There’s nothing terribly surprising about the Greens getting to this position. Although their support varies by state (Tasmania clearly the best, followed by Victoria and WA), they’ve had a pretty solid base in every state for some time, so no state looked unwinnable. In 2007 they collected three states and were very unlucky to miss out on a fourth; last year they all came good.

Whereas single-member districts typically lock out the smaller parties, the senate — despite its lack of one-vote one-value — generally returns something close to a proportionate result. As long as they can continue to command around one-eighth of the vote, the Greens will probably continue to hold about one-eighth of the seats.

Since last year’s election, many pundits have made a habit of proclaiming, or just assuming, that this is the Greens’ high-water mark: that they can only go downhill from here. That chorus became more intense after the last Victorian election, when the Liberals directed preferences to Labor ahead of the Greens, thus depriving them of any seats in the state lower house.

But in the senate the Greens mostly depend on their own votes, not on preferences. Only in New South Wales would their victory last year have been jeopardised if the major parties had preferenced against them. And the fact that Greens senators will often decide the fate of legislation gives the major parties a strong incentive not to antagonise them too much.

So should we get used to the Greens being arbiters in the senate on a more or less permanent basis? Maybe, but there are two reasons for caution.

First, balance of power doesn’t just depend on how many seats you hold, it depends on the relative position of the other forces. The Coalition’s underlying strength, however, is greater than Labor’s: if it could reverse its poor performance in Tasmania, or gain an extra seat or two somewhere to compensate (Queensland and WA are obvious possibilities), it would be well within reach of depriving Labor and the Greens of their combined majority.

Labor then would have to rely on either Nick Xenophon (if he’s still around) or the new DLP senator, John Madigan, in addition to the Greens, and we would be back to the more complicated dynamic of the past three years.

The second problem for the Greens is that with power comes responsibility. For the next three years there will be no one else to hide behind; whatever laws are made will have the express or implied endorsement of the Greens.

That could potentially work to their benefit: a party that is taking an obvious share in government will have a higher profile and will be less vulnerable to the charge that a vote for it is “wasted”. Many who sympathise with the Greens’ outlook could be attracted when they see them actually doing things rather than just talking.

But there will be major risks as well. Not all Greens voters will be happy with the inevitable compromises that will be made; a bigger Greens party room means more room for dissension and personality clashes; increased scrutiny could bring to light a range of policies that will alienate prospective supporters; and the Greens’ enemies, already frothing at the mouth, will be stirred to even greater efforts.

Whatever the vagaries of our electoral system (and it has plenty), the main thing determining the future of the Greens will be whether people keep voting for them. And that in turn will depend a lot on how they handle their new position of power.