Labour's support has sunk nearly six points and it is polling only 29.5 per cent in the Herald-DigiPoll survey.
The popularity of leader David Cunliffe has fallen by almost the same amount, to 11.1 per cent. That is worse than the 12.4 per cent worst rating of former leader David Shearer.
National could govern alone with 50.8 per cent if the poll were translated to an election result.
The popularity of John Key as Prime Minister has climbed by 4.6 points to 66.5 per cent. That is his best rating since the election but not as high as he reached in his first term when he often rated more than 70 against Phil Goff.
The increases in support for National and the Greens since December put them at their highest ratings since the 2011 election.
Crashing below 30 per cent will be a bitter blow for Labour six months before the election. No party in the six elections held under MMP has been able to lead a government without polling in the high 30s or in the 40s.
The Greens are up 2.3 points to 13.1 per cent and with Labour would muster a combined 42.6 per cent.
New Zealand First is down slightly to 3.6 per cent but leader Winston Peters' ratings as preferred Prime Minister at 6.5 per cent suggest the party could still top the 5 per cent threshold required to get MPs under MMP without requiring an electorate seat.
Other polls have shown a decline in Labour's fortunes this year but today's is the first to have Labour in the 20s since Mr Cunliffe took over the leadership from Mr Shearer in September last year.
In the DigiPoll survey in the immediate after-glow of Mr Cunliffe's election, Labour polled 37.7 per cent and had enough support parties to form a government.
But Mr Cunliffe had a halting start to the political year with controversy over baby bonus details in his state of the nation speech and publicity over donations to his leadership campaign.
Polling began on March 6, in the midst of the fallout over his use of trusts for donations.
But it continued through last week when Mr Key condemned minister Judith Collins for her failure to declare a dinner in Beijing with her husband's business associates.
Among the smaller parties, Act under new leadership is up marginally and registering enough support for one MP.
The Conservatives with 1.3 per cent would bring in two MPs if they won an electorate seat. Mana and the Maori Party are slightly down, but the leaders of both are expected to keep their electorate seats.
The percentage of voters who think the Government is moving in the right direction rose to 52.9 per cent from 46.6 per cent in December and the number who think it is not has fallen from 43.2 per cent to 34.8 per cent.
After seeing the poll results last night, Mr Cunliffe said that the issue of the donations and the trust had affected the party's polling but he did not believe it would be lasting.
"It hasn't been the easiest couple of weeks but I'm sure we will bounce back off that."
He believed the gap would close as the election neared.
Mr Key said the poll was a confirmation that a majority of New Zealanders believe the country is heading in the right direction "but clearly there is a lot more work to be done if we are to create the jobs and increase the living standards that New Zealanders want to see".
Asked if the issue of Mr Cunliffe's of Ms Collins non-declarations would have affected the poll, he said: "Voters weigh up a great many factors when considering who to support but I continue to believe the strongest motivation is when a political party is focused on the issues that really matter to voters."
The poll of 750 eligible voters was conducted between March 6 and March 16.
The party vote results are of decided voters only: National 50.8 (up 4 points); Labour 29.5 (down 5.9); Greens 13.1 (up 2.3); NZ First 3.6 (down 0.3); Conservatives 1.3 (up 0.6); Act 0.8 (up 0.8); Maori Party 0.2 (down 1.1); Mana 0.1 (down 0.8); and United
Future 0 (no change). The undecideds were 11.4 per cent.
Preferred Prime Minister results: John Key 66.5 (up 4.6); David Cunliffe 11.1 (down 5.4); Winston Peters 6.5 (down 0.8); Russel Norman 4.5 (up 1.1); Helen Clark 3.3 (up 0.1); David Shearer 1.6 (up 0.8); Shane Jones 1.1 (up 1.1).
A total of 11.4 per cent were undecided. The margin of error is plus or minus 3.6 per cent.
Devil in the detail
The Herald-DigiPoll survey shows:
AUCKLAND VS REST OF THE COUNTRY
* National is a little bit more popular in Auckland (53.8 per cent) than the rest of the country (48.8 per cent).
* Labour is less popular in Auckland (26.7 per cent) than it is in the rest of the country (31).
* The Greens have the same level of popularity in Auckland (13.4 per cent) as the rest of New Zealand (13) .
*New Zealand First is a lot less popular in Auckland (1.6 per cent) than the rest of the country (4.49).
MEN VS WOMEN
* National is supported fairly evenly among me (50.8 per cent) and women (50.7 per cent).
* Labour is better supported by women (31.5 per cent) than men (27.1 per cent).
* The Greens are fairly evenly supported by men (12.6 per cent) and women (13.5).
New Zealand First is more favored by men (5.5 per cent )than women (2).
YOUNG VS OLD
* National is most favoured in the 65 plus age group (60 per cent) compared to those aged 40 to 64 (53.2) and those aged 18 to 39 (41.3).
* Labour is most heavily favoured among the younger 18 to 39 voters (34 per cent) compared 40 to 64 year olds (28.4) and 65 plus (23.7).
* The Greens are most popular among the younger 18 to 39 voters (20.6) compared with 40 to 64 year olds (10.4) and 65 plus (7.4).
* New Zealand First is more favoured in the 65 plus age group (6.7 per cent) compared with the 40 to 64 years olds (3.4) and the 18 to 39 year olds (2.3).
PREFERRED PRIME MINISTER - MEN VS WOMEN
* Prime Minister John Key is more popular with men (70.4 per cent) than women (62.7 per cent).
* Labour leader David Cunliffe is a bit more popular with men (12.3 per cent) than women (10.1).
* NZ First leader Winston Peters is a little less popular with men (5.9 per cent) than women (7.1).
* Greens co-leader Russel Norman is a little less popular with men (4 per cent) than women (5.2).
PREFERRED PRIME MINISTER - AUCKLAND VS THE REST OF THE COUNTRY
* Prime Minister John Key is a little more popular in Auckland (68.9 per cent) than the rest of the country (65.4).
* Labour leader David Cunliffe is a little more popular in Auckland (12.4 per cent) than the rest of the country (10.4).
* NZ First leader Winston Peters is a lot less popular in Auckland (4.1 per cent) than the rest of New Zealand (8).
* Greens co-leader Russel Norman is a little less popular in Auckland (3.6 per cent) than the rest of New Zealand (4.9).