Labour leader Phil Goff says his party is prepared to discuss a capital gains tax with the Government, opening the way for an accord on a tax that has long been a political hot potato.
Mr Goff said the party's bottom line would be that the tax would not apply to family homes.
This means it would effectively target investment properties.
The issue is being considered by a Government tax working group, and neither Prime Minister John Key nor Finance Minister Bill English will rule it out.
A capital gains tax on property investment is seen as one way to reduce the tax advantages of rental housing, curb house price inflation and send investment into productive sectors of the economy.
The tax review group is due to report back this year.
Mr Goff said Labour would wait to see what it came up with, but was prepared to work with the Government if it had a persuasive case.
"We are utterly against a capital gains tax on people's homes. But if the Government has other ideas, we'll keep an open mind."
Mr English would not comment on Mr Goff's offer last night.
He and Prime Minister John Key have made it clear National does not favour a capital gains tax, but they do not want to place any limits on what the review group comes up with.
Labour will not support an increase in GST, another proposal the group is assessing.
In his speech to the Labour conference at the weekend, Mr Goff also attacked "price-gouging" by state-owned power companies, saying the party got it wrong in failing to fix the system while in government.
He said power companies should not be geared to make a profit and pay dividends to the Government as it was in effect a tax which fell on those least able to pay it.
Mr Goff cited Mighty River Power, which this month return $230 million to the Government, saying if a company could feed back profits like that, then New Zealanders' power bills were too high.
A Labour government would stop demanding dividends beyond what was needed for new generation.