The party has lost 5.5 percentage points since March, and Mr Shearer is down 6.1 points as preferred Prime Minister.
National's support has barely moved and it is still polling high at 48.8 per cent of decided voters.
Prime Minister John Key is preferred Prime Minister for 65.2 per cent, up 2.6 points on his last rating.
If the poll were translated to votes, National would not be able to govern alone.
Either National or Labour would be able to form a government.
But Labour would need four other parties, and National would have more options including being able to form a government with only New Zealand First support, only Maori Party support, or support from Act and United Future.
Mr Shearer has been Labour leader for a little over 18 months, after beating former finance spokesman David Cunliffe in a contest involving the party membership.
Today's poll could put added pressure on Mr Shearer's leadership, which has been attracting stronger criticism lately from the remnants of Mr Cunliffe's support on the left than from the right.
Asked on TV3's Firstline this morning if the poll result meant he was on the right track, Mr Shearer admitted the poll was a surprise.
"Polls go up and down, I'm a bit surprised by this poll."
He denied it was time to rethink Labour's strategy or policy and said he had no intention of stepping down as the party's leader.
"We're getting way ahead of ourselves, it's one poll one and a half years out from an election," he said.
Mr Key has taken to taunting Mr Shearer in Parliament about being under the control of deputy leader Grant Robertson.
But more voters see Mr Cunliffe as a successor should Mr Shearer no longer be in contention.
Asked who would be best to replace Mr Shearer if he left politics, 31.8 per cent supported Mr Cunliffe and 16.7 per cent Mr Robertson.
The dark horse is Andrew Little, a first-term list MP and former national secretary of the Engineering, Printing and Manufacturing Union, who had 13.5 per cent support as a possible replacement.
Eleven per cent wanted someone else and 27 per cent did not know or refused to say.
Among Labour supporters, 37.7 per cent supported Mr Cunliffe if Mr Shearer weren't there, 19 per cent Mr Robertson and 14 per cent Mr Little.
Labour's party vote support appears to have gone to New Zealand First and the Greens.
New Zealand First's support rose above the 5 per cent threshold, and support for Winston Peters as preferred Prime Minister is up from 4 per cent to 6.4 per cent.
Party vote figures in the poll are: National 48.8 (up 0.3), Labour 30.9 (down 5.5), Greens 10.5 (up 1.5), NZ First 5.1 (up 2.6), Maori Party 1.8 (up 0.7), Mana 0.5 (no change), United Future 0.3 (up 0.3) and Act 0.2 (up 0.1). The Conservatives gained 2.65 per cent of the vote at the last election but no MPs polled 1.5 (up 0.2).
Support for Mr Shearer rose in March to 18.5 per cent when National was mired in issues such as the failure of the Novopay pay system for teachers, the Solid Energy crisis and the partial sale of Mighty River Power.
Since the last poll, the Government has lost list MP Aaron Gilmore and passed its Budget, but a lot of the political focus since then has been on United Future leader Peter Dunne.
Mr Dunne resigned as a minister as the chief suspect in the leak of a confidential report.
Party vote results and preferred Prime Minister results are of decided voters only.
On the party vote questions, 11.9 per cent of poll respondents were undecided.
- Audrey Young, NZ Herald
