Watching Christine Milne’s speech at the National Press Club this week, your humble correspondent suddenly heard the lilting strains of Jeff Buckley’s Gen-X break up anthem "Last Goodbye" drifting through his brain. "Well, maybe it’s just because I didn’t know you at all" runs one of the more pointed lines.

The Greens-Labor romance is over, at least for the time being. Like many break ups, the initial shock has been followed by bitter recriminations and some over-sharing on social media. ALP types and Greens seem to be relishing the opportunity to finally speak their minds, laying into each other over who’s at fault, and who is the better protector of the environment. So far, no-one has asked for their toothbrush back. But it can only be a matter of time.

In her speech, Greens leader Christine Milne argued that the Government’s decision to open up the Tarkine forest for mining and the mess made of the Minerals Resource Rent Tax were the triggers for the break up. She also mentioned coal seam gas drilling, subsidies to the fossil fuel sector and the reduction of benefits payments to sole parents.

"By choosing the big miners, the Labor Government is making it clear to all that it no longer has the courage or the will to work with the Greens on a shared agenda in the national interest," she said in her speech on Tuesday.

For her part, Prime Minister Julia Gillard has greeted the separation with equanimity, even glee. "At the end of the day, the Greens party is fundamentally a party of protest rather than a party of government," she told a media conference yesterday. "The Greens party is fundamentally a party that would prefer to complain about things than get solutions."

The original Greens-Labor agreement, reached in the heady weeks after the August 2010 election, was one of the key instruments delivering power to Julia Gillard for a second term. It’s worth revisiting at this juncture, if only because the later mythology about the Greens as some kind of radical fifth column dictating the actions of the Gillard government has proved so resonant.

For all the later rhetoric from Andrew Bolt and his kin, the agreement was quite modest. It was mainly framed around improving parliamentary process, including election funding reform, better consideration of private members’ bills and the establishment of a Parliamentary Budget Office. The policy section of the agreement is right at the end, and lists only four issues: a carbon price, dental care, a feasibility study on high speed rail, and a full parliamentary debate on the war in Afghanistan.

Nowhere in the agreement is the mining tax mentioned. Neither is federal regulation of coal seam gas drilling, or welfare policy.

Did Labor deliver on the agreement?

Yes, actually. As we know, a carbon price came into effect in July last year, after the Clean Energy Bill was passed. The debate on Afghanistan was held in 2010 and a report into high speed rail was released in 2011. The Parliamentary Budget Office was established. And last year, the Government poured $4 billion into dental care for children and low income earners.

In other words, the Greens got everything Julia Gillard agreed to back in September 2010. If relations have soured with Labor since then, it is not because Labor walked away from the agreement. It is because the Greens have fallen out of love.

So is the break up justified? It does rather seem like a divorce of convenience. After all, the Greens will continue to vote with Labor on supply and confidence motions on the floor of Parliament, in effect keeping the ALP in government. While both parties are attacking each other publicly, the reality is that the Greens will continue to vote with Labor on the majority of issues for the foreseeable future.

The Greens are keen to attack Labor from the left in the upcoming election, a tactic made more difficult when the two parties are in a form of government coalition. For its part, Labor is delighted to be free of the minor party, and its troubling tendency to extract policy concessions from a government that would prefer the flexibility to pivot to the right.

By way of illustration, AWU boss Paul Howes was enjoying himself on Lateline last night as he made the pragmatic point that the Greens were as much Labor’s opponents as the Liberals. "In the cut and thrust of Australian politics I believe the Australian Labor Party should act to beat their political opponents," he told Tony Jones.

Political analysts — including many on the conservative side of politics — have suggested that the break up is all for show, in order to differentiate the two parties in the run up to the September election.

This is debatable. Whatever the right wing perspective, we should not underestimate the very real hostility between the two parties, particularly at the local level in places where the Greens genuinely compete for Labor votes, such inner-city Sydney and Melbourne. There is no love lost between Anthony Albanese and Lee Rhiannon, for instance, who regularly face off at polling stations in the progressive belt of Sydney’s inner west.

The same can be said for the Greens sole lower house MP, Adam Bandt, and his many ALP enemies in his seat of Melbourne. In fact, demographic change and the slow hollowing out of tribal party loyalties means that in certain enclaves of the big cities, the Greens and Labor are natural opponents.

The real motive for the breach may be about the Greens’ position in the new Parliament at the end of this year. After all, given Labor is likely to lose the election, the minor party may soon find itself back on the cross benches while an Abbott government sets about dismantling Australia’s carbon policies. Breaking with Labor now allows the Greens to set themselves up as a more principled and effective opposition than Labor, which is likely to descend into internecine warfare for some time in the wake of what may well be a devastating defeat. On current polling, the Greens will likely lose Adam Bandt’s lower house seat, but retain between eight and eleven Senators. They will remain a significant force in the upper house.

As Tad Tietze writes today, there is a natural tension in the political position of the Greens on the left of Labor. Julia Gillard is of course partly correct when she points to the Greens’ roots in environmental protest; the party’s base remains passionately committed to anti-market policies that are profoundly alienating to a large proportion of the electorate. But the Greens as a parliamentary party can achieve few policy outcomes without the help of the bigger and older progressive party to their right.

And so it has proved over the last two and half years. Acting in partnership with Labor, Christine Milne was able to drive Labor towards a carbon policy that is significantly to the left of the original Carbon Pollution Reduction Scheme proposed under Kevin Rudd. She also won agreement on a $10 billion clean energy investment fund, which would almost certainly not exist without the Greens’ influence. The Greens can also point to their wins on the PBO and on dental care. These are all policies that both Greens and Labor voters broadly support. Logic and recent history therefore suggests that the best outcome for voters interested in more left-leaning policies is a Greens-Labor coalition, not a Greens-Labor break up.

As an independent third party sitting on the far left of an Australian political landscape that is tilting right, the Greens face some difficult decisions. By returning to the comfortable status of principled protest, the party can protect its base and continue to demonstrate its independence. But the move also risks accusations of irrelevance.

Nor is the split necessarily a good outcome for Labor. Shorn of the need to keep the Greens happy, the ALP is likely to slowly drift back towards the centre and centre-right. This offers opportunities to tailor policies that can combat the Coalition in key marginal seats in the outer suburbs. But it also accentuates Labor’s own dilemmas of retaining its educated, progressive inner-city support.

It’s not easy keeping Greens and Laborites happily married. But, like most relationships, the long term gains of sticking together are likely to outweigh the irritations and frictions of a shared life.