When nominations closed for the New South Wales state election last week, there was a surprise in store: a large number of candidates nominating for small parties on the left.

We learned more this week, when pre-poll voting opened and we got the first hint of how these parties are preferencing. A group of minor parties have apparently arranged a preference swap among themselves, as well as with Labor and the Greens.

Over the last decade, Australian politics has seen a big increase in support for small parties, but most of these new parties have emerged on the right. Yet this election has seen a huge turnout from parties competing with Labor and the Greens on the centre-left of politics, some of whom could have a chance of picking up upper house seats.

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The how-to-vote cards released by the parties on Monday appear to show a preference-swap arrangement between these progressive minor parties, the Greens and Labor.

All three of these parties could be considered to be rivals to the Greens and they are contesting significant numbers of lower house seats in this state election. Sustainable Australia is running 55 candidates, the Animal Justice party is running 48 candidates and Keep Sydney Open is running 42 candidates.

While the Greens have been preferenced highly by two of these three parties, they are in danger of having their support base chipped away, and could potentially lose out in the upper house race to one of these rivals.

Keep Sydney Open was founded primarily to oppose the Sydney lockouts, but has adopted a broader platform that is partly libertarian but also overlaps a lot with the Greens. Some would question Sustainable Australia’s progressive credentials, but its anti-immigration stance is about environmental impact rather than race, and its positions on development and immigration overlap substantially with some parts of the Greens base. Indeed a number of their candidates have previously stood for the Greens. These parties are very different to rightwing minor parties like the Shooters, Christian Democrats and One Nation.

None of these lower house candidates will have a chance of winning a seat, yet it shows a certain organisational capacity to sign up a large number of candidates, as well as finding the money for nomination fees.

Another challenge to the Greens is coming from Jeremy Buckingham, who until recently was a Greens member of the upper house. Buckingham resigned from the party after accusations of sexual assault, which he strongly denies. He is not running any lower house support candidates, and will suffer from the lack of a party name on the ballot or any party organisation to fund and support his campaign.

Socialist Alliance and the Voluntary Euthanasia party are also contesting the legislative council election, with Socialist Alliance also running in a handful of lower house seats.

These parties are running most of their candidates in areas with a higher Greens vote, particularly in the eastern half of Sydney. All three of these parties are running candidates in each of the three Greens-held electorates.

With a growing field of parties and candidates running on the left, preferences will become more important.

Under the NSW upper house voting system, voters choose their own preferences above the line. Most voters usually just put “1” next to their preferred party, and their vote thus exhausts without flowing to any other party. At the 2015 election, 83% of voters cast a single vote above the line, while a further 15% numbered multiple boxes above the line and fewer than 2% voted below the line.

Yet preferencing is more popular among minor parties, with most flowing among the same group.

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Lower house preferences also suggest deals between Labor and progressive minor parties. The Greens are preferencing Labor over the Coalition in all 93 electorates. Keep Sydney Open is preferencing Labor over the Coalition in every seat it is contesting. Animal Justice appears to be doing the same in the handful of seats where how-to-votes have been sighted. In return, these three parties have received high-ranking preferences from Labor in the upper house.

Preference recommendations are one thing, but they won’t matter unless these parties can hand out how-to-vote cards. Labor and the Greens will have a strong presence on polling booths, but the smaller parties won’t. Their ability to hand out how-to-votes and advise their voters on preferences could have a big impact on the last few seats in the upper house, where preferences could be crucial.

The current presence of some of these parties may not be permanent, and we don’t know whether these parties would be able to work together if they won seats in parliament. But there is no doubt that there is a growing surge of parties threatening the Greens’ flank and will continue to be a story in coming elections.