The National Party is comfortably ahead in the latest 3 News opinion poll, but the Labour Party has made up some ground.
Support for National was unchanged on 48%, while Labour's support was up two percentage points to 37%.
In May, National had a lead of 15% in the poll, but that is now down to 11%.
The findings mirror similar trends in the One News and Roy Morgan polls.
Radio New Zealand's political editor says Labour Party insiders believe the gap between the two main parties will continue to narrow as the election, less than three months away, draws nearer.
In the 3 News poll: the Green Party recorded 6% support, New Zealand First 3%, ACT and the Maori Party 2% each and United Future 1%.
House to resume
Parliament resumes on Tuesday for what is expected to be its last three-week sitting.
Parliament will next come back on 23 September but the House cannot sit any later than 6 October so it could go for only two weeks at the most and Radio New Zealand's political editor says there is a lot to get through.
The Government hopes it will get support on Tuesday from the Greens and New Zealand First for its legislation on the emissions trading scheme.
Radio New Zealand's political editor says it is expected - although it is not guaranteed - that both parties will do so.
It is also understood that the Greens will not agree to go into urgency, which puts more pressure on the Government to get the bill passed by the time Parliament rises before the election.
The Government also wants to get a big central North Island forestry deal through, plus at least first readings of a number of other settlements under the Treaty of Waitangi.
In addition, there is a bill on the regulation of financial advisers and the Reserve Bank Amendment Bill, which gives Reserve Bank power to supervise non-bank financial institutions, such as finance companies.
Parliament also has to set aside time for valedictory speeches for those MPs who are retiring and some changes to standing orders.
A report by the Privileges Committee into New Zealand First leader Winston Peters will have to be debated - a considerable workload to get through in three weeks.