The Coalition’s leadership implosion is continuing to deadweight the Morrison government in the polls, with Labor in front on a two-party-preferred vote of 56% to 44%, according to the latest Newspoll.

Two weeks after Scott Morrison was declared the victor in a poisonous three-way leadership contest that dispatched Malcolm Turnbull from the prime ministership, Labor is in an election-winning position similar to the vote the opposition commanded in 2007 when Kevin Rudd took government from John Howard.

Morrison is ahead of the Labor leader, Bill Shorten, as preferred prime minister 42% to Shorten’s 36%. The Guardian Essential poll a fortnight ago had a similar reading, with Morrison on 39% and Shorten on 29%, but Labor stretching its lead over the Coalition to 10 points.

The new prime minister’s approval rating is 41% and his disapproval rating is 39%. Shorten’s approval is on 37% and his disapproval is on 51%. The poor result is the Coalition’s 40th straight loss in the Newspoll.

With federal parliament set to resume for the first time with Morrison in the top job, and the government still plagued by internal dissent and by leaks – the poll result won’t reassure Coalition MPs nervous about a voter backlash as a consequence of the leadership turmoil.

On the ABC on Sunday, the new deputy Liberal leader and treasurer, Josh Frydenberg, ducked questions about why Turnbull had been sacked by his colleagues. Frydenberg said that analysis should be left to commentators.

Pressed to give an answer, he said: “Well, every Australian is more interested in actually getting those essential services in health and in education and in the NDIS that they need, in getting this continuous strong jobs growth, in getting lower taxes, which will be reversed under a Shorten-led government.”

The poor poll result follows a disaster for the Liberals in New South Wales at the weekend in the Wagga Wagga byelection. The Liberals appear to have lost the state seat for the first time in six decades.

The NSW premier, Gladys Berejiklian, said federal factors influenced the result. “The overwhelming message I was getting is that people were sick of politicians fighting amongst themselves and sick of the perception that politicians were in it for themselves and not the community.

“And the circumstances which forced the previous member to resign, plus what happened at a different level of government, exacerbated those feelings that people had.”

A new Guardian Essential poll is due on Tuesday. Leadership coups in the past have delivered the major parties an average bounce in the primary and two-party-preferred measures of 3%, according to that data.

This positive trend was consistent from Kevin Rudd to Julia Gillard, Gillard back to Rudd, and Tony Abbott to Malcolm Turnbull – so the switch to Morrison is the first time an incumbent government’s position has deteriorated as a consequence of a change.

Morrison has worked assiduously over the fortnight since taking the leadership to stabilise the government after the turmoil, dump unpopular measures, and introduce himself to the voting public with a headland speech drawing on the Liberal party’s traditional values.

But the government faces pressure on a number of fronts. It will need to defend Turnbull’s Sydney seat of Wentworth at a looming byelection to defend its slender majority in the House of Representatives, with polls suggesting that voters are in a mood to punish the government.

The fortnight of parliament opens with the home affairs minister, Peter Dutton, facing political pressure about the use of his discretionary power to grant au pairs tourist visas, and with Labor intending to press key figures about their role in the leadership ruckus.

Labor will also attack the government over its decision to dump the national energy guarantee, leaving the Coalition without a credible emissions reduction policy to take to the next federal election.