Bill Shorten’s net approval rating has fallen to an all-time low in a recent opinion poll, but Labor remains a nose ahead in the two-party preferred stakes.

The Coalition has edged up one percentage point since last month, sitting at 49% to Labor’s 51% in the latest Newspoll, published in The Australian newspaper.

Labor’s primary vote has fallen from 37% last month to 34% this month, while the Coalition has fallen one percentage point to 40%. The Greens’ primary vote is up to 14%, their highest primary vote since September 2014.

But voters appear to be dissatisfied with the performance of both leaders. Both prime minister Tony Abbott and opposition leader Bill Shorten have seen a dip in ratings.

Shorten’s net approval rating is his lowest ever, with 28% approving of his performance, and 54% disapproving, giving him a net approval rating of -26%.

Abbott’s net approval rating is only marginally better. His satisfaction rating is 34% and disapproval rating is 56%, giving him a net approval rating of -22%.

The net approval ratings have plunged since last month, when Abbott and Shorten sat at -15% and -18% respectively.

Abbott remains the preferred prime minister, on 41% to Shorten’s 38%, but 21% of respondents remain uncommitted either way.

The survey of 1,086 people was conducted over the weekend, after claims of Australian authorities paying people smugglers emerged and Shorten said he would front a royal commission into trade union corruption over questions relating to his time in the Australian Workers Union.