We saw such an alliance when the Coalition and Greens joined forces to change the system of Senate voting in an apparent attempt to rid us of micro-party senators and perhaps help the Coalition get a Senate majority.

This week the High Court heard a challenge to it. At time of writing it had not made a decision. My guess is that it will dismiss the challenge (giving reasons later) in time for Prime Minister Malcolm Turnbull to take the fateful journey along Dunrossil Drive to Government House to advise the Governor-General to dissolve both Houses of Parliament and have an election.

But who knows what a court might do?

The Constitution has a lot to say about electoral matters and the temporary alliances of political parties can use it to help their cause or blunt the cause of their opponents.

We have not had a double dissolution for nearly 30 years, the last one being in 1987. After a double dissolution, the Constitution provides that we must get back to a position where half the Senate is elected every three years, so the Senate rotates every six years.

It means that half of the Senators from the states will get three-year terms and the other half six-year terms. Territory senators' terms are concurrent with the House of Representatives.

So which senators get the short terms and which the long?

In an ideal world, there would be a definitive and fair method of selection. But the world of constitutional law is not an ideal one.

The Constitution merely provides: "After each first meeting of the Senate following a dissolution thereof, the Senate shall divide the senators chosen for each state into two classes, as nearly equal in number as practicable; and the places of the senators of the first class shall become vacant at the expiration of three years, and the places of those of the second class at the expiration of six years."

So it is a matter for the senators themselves, and it will remain that way unless there is a constitutional amendment – very unlikely. In short, the senators (read a majority of them) can force whatever selection method they like to determine which senators get the long terms and which get the short.

As it happens, the Parliament has provided in the Commonwealth Electoral Act a very fair method to determine the matter, but it is not binding on the senators because this clause in the Constitution, unlike many other clauses, does not begin with the crucial words "until the Parliament otherwise provides . . . ".

The Electoral Act method is to do a recount of all the votes as if the election were for just six senators in each state, not 12, and the six senators thus notionally elected would get the long terms. All very fair and democratic.

But after the 1987 election, Labor and Democrat senators did the dirty. They just ignored the Electoral Act and resolved, as the Constitution permits, that the order of election would determine who got the long terms.

The difference is critical. During the count, those who get a quota are elected before those who get a seat after distribution of preferences. So if the Greens or Xenophons got two clear quotas, they would both be elected before the distribution to second-on-the-list major-party candidates.

Under the Electoral Act method, only one Green or Xenophon candidate would win one of the notional six seats that would translate into a six-year term.

You watch. The major parties will form an alliance of convenience either with each other or with a minor party to cook up a method so they get as many long-term senators as possible.

They could draw lots if they wanted. Or the major parties could be utterly skulduggerous and resolve something like that, where a party gets three or more senators elected, the first three of its senators will get the long terms.

That would give Labor and the Coalition all the long-term senators. It would mean that in the 2019 election for half the senate, all the minor-party senators would face re-election.

The Greens would be reduced to five or six senators rather than the nine or 10 it gets in the usual rotation.

On a related matter, the determination of the major parties to rid the Senate of minor-party representation appears permanently doomed (skulduggery aside, of course).

Support for the major parties has fallen so significantly in the past 35 years, that they cannot prevent one or two non-majors to be elected in every state at a half-Senate election and two to four in every state at a double dissolution.

Further, under the new optional-preferential voting system, a lot of votes will exhaust during the count. That is, before the last one or two senators are elected, a vote expresses no preference for any of the live candidates.

It is likely therefore that the last candidate elected will be elected on a lot less than the quota of 7.7 per cent of the vote after preferences. Maybe as low as 2 per cent.

It may be that enough votes exhaust that the last two senators are both elected with less than a quota. That will happen if more than a quota (that is 7.7 per cent) of votes exhaust.

Given Australians' proclivity for political apathy and laziness, that is very likely.

So the idea that the new voting system will clean out the micro-parties and independents may be a bit optimistic from the view of the major parties.

The latest Morgan poll has Labor on 32.5 per cent (4.2 quotas); the Coalition on 40.0 per cent (5.2 quotas); Greens 13.5 per cent (1.75 quotas); Independents 13.5 per cent (1.75 quotas).

That translates (on average across the states) to four Labor senators; five Coalition; one Green and one Independent and a dogfight for the last seat, but favouring the Greens and Independents. Obviously, it will vary across the states, but you get the picture – an average of three cross-bench senators per state. That's the same as we have now.

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