Electoral success for the Greens in lower house seats is no longer the novelty that it was just a few years ago, but last week’s victory in the Victorian state seat of Prahran was particularly sweet for the party in being the first it had ever achieved in a Liberal-held seat.

To some of the Greens’ most ardent adherents on social media, the win suggested the beguiling prospect of future success in areas never before contemplated. “Today Prahran, tomorrow the world” might be a bridge too far, but some at least dared to contemplate the prospect of “today Prahran, tomorrow Hawthorn and Kew”.

Previous successes had exposed the Greens to the charge that the party’s efforts were hampering the main game of keeping the conservatives out of office by obliging Labor to divert resources to once-safe inner-city seats.

By contrast, Prahran delivered a seat that could not have been won by Labor, which lost the two-party preferred count against the Liberals by the tiny margin of 18,580 to 18,555.

These totals were made redundant by a second nail-biting result during the preference distribution, in which Sam Hibbins of the Greens edged out Labor’s Neil Pharaoh to take second place by 9979 votes to 9948.

A result that came down down to the balance of support between Liberal and the Greens left psephological observers in uncharted waters.

Copious data is available on how preferences divide between the Coalition and Labor, with the Australian Electoral Commission having published exact breakdowns for every minor party and independent candidate since 2004.

The Greens’ successes in inner-city Sydney and Melbourne have also established how Liberal preferences behave, with three-quarters going to the Greens when the how-to-vote card has them ahead of Labor, and two-thirds going to Labor when it hasn’t.

But until last week, the division of Labor votes between Liberal and the Greens had been a purely academic consideration, on which next to no data was available. Before 2011, the Greens were able to make the final count in only in the most heavily Left-leaning of electorates. That changed with Labor’s collapse at the last state elections in New South Wales and Queensland, but the optional preferential voting system in these states meant around 60% of Labor voters failed to allocate preferences, as they would have been obliged to do in Victoria.

By the reckoning of election observer Kevin Bonham, the one precedent of a Liberal-versus-Greens contest under compulsory preferential voting was in the Adelaide Hills seat of Heysen at the South Australian election in March, a safe Liberal seat held by the party’s former leader Isobel Redmond. On that occasion, 20% of Labor voters allocated their preference to Redmond against the direction of the party’s how-to-vote card — which tend to be more closely adhered to in South Australia, owing to the state’s unusual practice of putting them on display in polling booths.

With only that to go on, it seemed prior to the preference count in Prahran last Tuesday that the Greens had a high mountain to climb to secure the 83% of preferences needed to draw clear of Liberal member Clem Newton-Brown. So it was something of an eye-opener when fully 88% went Hibbins’ way, securing him what in the circumstances was a surprisingly comfortable victory by 277 votes.

With Labor preferences locking in so emphatically behind the Greens, it now appears that any circumstance where the Greens can sneak ahead of them offers a serious prospect for victory if the Liberal primary vote is more than a few points shy of 50%.

However, thoughts of a Greens march through the wealthy bastions of the Liberal Party run into the wall of the Liberal primary vote, which remains too high for them to be challenged by the Greens or anywhere else.

What Prahran was able to offer was a combination of inner-city sensibilities at the southern end of the electorate — in Prahran itself, a focal point of the gay community, and the bohemian environs of St Kilda East — and the affluence of South Yarra and western Toorak, which served to dampen Labor’s vote to the extent that the Greens were able to surpass it.

North of the border, a similar conjunction of circumstances can be identified in a few places in Sydney, and perhaps even in the Byron Bay-region seat of Ballina. But the prospect of a Prahran scenario playing out at a New South Wales state election is effectively ruled out by optional preferential voting, which would have starved the Greens of the decisive Labor preferences if it had been in operation in Prahran.

At federal level, though, the description just offered of Prahran sounds rather a lot like Malcolm Turnbull’s electorate of Wentworth, which spans exclusive Vaucluse, the gay enclave around Paddington and the Left-leaning domains of Randwick and Bronte.

Turnbull’s epic personal vote is such that the Liberals have nothing to fear for as long as he is there to defend it. But the seat was looking increasingly marginal not so long ago, and should Turnbull decide to move on — as he briefly did after losing the leadership at the end of 2009, only to change his mind shortly afterwards — the Greens will have something to play for, particularly if the broader election result is a poor one for the Liberals.