The government has lost further ground in Newspoll, now trailing Labor 46-54% in two-party terms, in the wake of the crisis over citizenship.

This is the 18th consecutive Newspoll in which the government has been behind. The two-party fall comes after several polls in which the Coalition trailed 47-53%.

The early part of the poll fortnight was dominated by the issue of the postal vote on same-sex marriage. Then the declaration of Deputy Prime Minister Barnaby Joyce that he had been informed he was a New Zealand citizen began a horror week for the government which ended with Fiona Nash, the deputy Nationals leader, announcing she had British citizenship.

Labor increased its primary vote by two points to 38%, while the Coalition fell one point to 35%. One Nation rose one point to 9%, equal with the Greens, who lost two points over the fortnight.

Malcolm Turnbull’s net satisfaction rating has plunged from minus 12 to minus 20 in the poll, published in Monday’s Australian. Bill Shorten’s net satisfaction also took a hit, deteriorating from minus 15 to minus 20.

Turnbull still has a significant lead as better prime minister – 43-33% – although the gap narrowed from the previous 46-31%.

The poll contains encouraging news for the “yes” case in the postal ballot, with 63% saying they would vote yes to the plebiscite question, compared with 30% who would vote no. More than two-thirds of people (67%) said they definitely intended to vote; another 15% said they probably would.

Nearly half (49%) said they were in favour of the postal plebiscite while 43% were opposed.

Asked whether parliament should provide guarantees in law for freedom of conscience, belief and religion if it legislated for same-sex marriage, 62% said yes and 18% said no.

The support for same sex marriage is strongest among younger voters, with 70% of those aged 18-34 in favour. It is lowest among those aged over 65, with only 49% supporting it.

The poll was of 1,675 people and taken between Thursday and Sunday.