Coalitions are primarily a marriage of convenience as each party to the arrangement would prefer to be governing alone. Though some coalitions are business as usual, like the Liberal-National coalition, they usually arise when neither partner has the numbers to form a majority in the parliament. It does not matter if they are like-minded, even sister parties, only the raw numbers matter.

Generally in a coalition the distribution of ministries is determined by the numerical balance between the two governing parties. In the case of Liberal-National coalitions that balance is revisited after each election regardless of the quality of the ministerial talent available. Every small change in the balance between the size of the two parties in parliament makes a difference.

The same should apply to Labor-Green ministries in the ACT. If the Labor-Green ratio of MLAs is about 3:1 then the ratio of ministries should be about the same. If it is closer to 2:1 then the same applies. Either way this should mean an increase in Greens representation in the new ministry.

The largest party in the parliament, such as ACT Labor now, always has options. When lacking a majority they can try to govern alone with support from the cross bench or they can form a formal coalition. Even with an absolute majority Jacinda Ardern is considering her options in New Zealand, to be in coalition or not, and Andrew Barr and Shane Rattenbury can do the same in the ACT.

Coalitions have several virtues. The prime one is stability and solidity. The majority in the parliament is guaranteed without having to revisit it every time a piece of legislation is put up.

Coalitions also directly represent a wider section of the community, bringing together supporters of the two parties. This reinforces their legitimacy in the community as a government representing the majority of voters. This applies to Labor and the Greens in the ACT because together they attracted a majority of votes (Labor 38 per cent and Greens 13.5 per cent) last Saturday.

Another attraction of coalitions is greater inclusiveness, something that Ardern has discussed in NZ when considering whether to invite the Greens into a coalition. When the parties largely represent distinct constituencies, urban-rural, young-old, union-non-union or different ethnic or faith communities, the government is stronger for having a greater mix inside the tent.

Coalitions also expand the talents available to a government in forming ministries. The smaller the assembly and the smaller the size of the largest party, the fewer members of parliament to draw on. The talent pool of ability and experience is limited in any parliament.

In an assembly of just 25 it is particularly limited. Finding eight ministers of sufficient expertise and experience is often difficult, especially when only members of the governing parties are in contention. This is not meant to diminish other MLAs. Skills differ. Ministers need specific intellectual, communication and management skills to do the job well. Some have a natural talent for it. Other members are naturally better suited to being campaigners and parliamentary representatives than government ministers.

All these factors add up to a strong case for the value of coalition governments. But there are also definite negatives.

The starting point for discussion of the negatives must be that coalitions are a marriage of convenience rather than love. They are a business arrangement bringing together parties with different cultures, compositions and driving forces. These parties are also electoral competitors, chasing the same voters, either centre-left or centre-right, and this can breed anger and resentment.

These differences clearly apply to Labor and the Greens. Labor has an older-style machine culture, built around factions and trade unions. The Greens have a participative, more inclusive newer-style culture rooted in social movements. Labor is more used to government and more practical, while the greens are more idealistic and aspirational.

These differences are brought together in a framework in which Labor, the larger party, is the more senior, and the Greens are the junior partner. That can create problems if the junior partner tries to be too pushy. Think Barnaby Joyce or John Barilaro in the Liberal-National context. Already Barr has indicated that the junior partner cannot dictate policies or even what ministries they occupy.

Both sides have their own priorities and must stick by their principles, but they must always be willing to compromise and to make concessions. That is problematic because even when an agreement can be reached within the government it might be unpopular with party members outside parliament who expect their own policies to be implemented.

In the ACT both Labor and the Greens will benefit from a smooth transition to a new coalition. But it may not be unproblematic, nevertheless. Not everyone in the two parties respects the other side or accepts that they are natural partners. The parliamentary balance between them has clearly changed, there are many new faces, and conflicting interpretations of the message the community has sent to the government.

The coalition has been relatively stable for the past four years, with Rattenbury playing a significant role. The two parties would be unwise to experiment with a new governing arrangement when they have the opposition on the ropes. They should stick with a winning recipe.