Community opposition to WestConnex appears to have been the decisive factor in the Greens' win in the inner city seat of Newtown, new polling shows.
Former transport spokeswoman Penny Sharpe suffered a heavy defeat, losing by nearly 40 to 60 per cent on a two-party preferred basis.
The loss of that seat, and failure to reclaim Balmain, has been the subject of much post-game analysis within the party and particularly its left faction, to which both failed candidates belonged.
Some party figures have argued the party needed to do more to attract progressive voters after Jenny Leong won a 10 per cent increase in primary vote.
But WestConnex appears to have been the decisive issue.
About a quarter of Newtown voters polled nominated WestConnex as their most decisive election issue, in a survey conducted for the Greens. Privatisation and the economy was ranked the next-most important issue by 18 per cent of voters.
About 54 per cent of voters nominated the Greens' Ms Leong as the most effective candidate on the issue, including a little less than a third of Labor voters. Only 26 per cent of voters thought ALP candidate Penny Sharpe would be more effective.
Many Newtown residents have protested plans for a major interchange in St Peters, near Sydney Park, where the new M5 East tunnel is planned to emerge. There are fears it will impact the local environment and traffic.
Ms Leong and the party advocated scrapping Westconnex entirely in favour of public transport.
Under Labor the M5 tunnel would have emerged around Port Botany, and while it would have built motorways on either side of the inner west the party would not have connected them with a tunnel.
But the stance appears to have won the party few votes.
Garth Montgomery, from the Westconnex Action Group, said his organisation had 27 volunteers handing out material at booths in Newtown and Heffron.
Mr Montgomery said the group declined a request from Labor to retract a flyer that noted the ALP would still build the motorway.
"There's absolutely no doubt our polling day efforts had a massive impact," Mr Montgomery said.
Mr Montgomery said Ms Sharpe had not shown up to anti-motorway protests including one in March that attracted 3000 demonstrators and Ms Leong and Lord Mayor Clover Moore.
Labor figures have also privately conceded the strength of Ms Leong's appeal to the inner-city electorate.
But Labor's stance on WestConnex also opened it to criticism from Premier Mike Baird that it was determining the path of infrastructure projects for political reasons.
Slightly more Newtown voters nominated coal seam gas as their biggest concern than health with both registering about 9 per cent.
The electorate lies next to St Peters, where community opposition to a coal seam gas well reached a peak about 2011. The government has since passed a law excluding drilling from within two kilometres of homes.
Ms Sharpe did not return calls on Thursday.
The Reachtel poll of 650 voters was conducted on Saturday and Sunday.
