Labour leader Phil Goff has suggested his party may re-negotiate an investment agreement with Australia if it wins this year's election.
Australian Prime Minister Julia Gillard arrives in New Zealand on Tuesday and will sign the investment protocol with Prime Minister John Key on Wednesday.
The deal is seen as the next step in bringing the two economies closer together.
But Mr Goff, a former trade minister, is cautious about it and warns the risk Australian investors might pose because of their investment capability.
He says Australian banks already take about $2.5 billion out of New Zealand in dividends.
However, Mr Key says the Government has considered the investment protocol carefully before agreeing to higher thresholds for Australian investors.
Australians will be able to invest up to $477 million in New Zealand without facing screening.
In turn, New Zealanders will be able to invest about $1 billion in Australia.
Radio New Zealand's political editor says Ms Gillard will not be too concerned. The trans-Tasman relationship has always been more important to New Zealand than it is to Australia.
The Council of Trade Unions says the investment deal with Australia would undermine attempts to keep control of New Zealand's strategic assets.
CTU economist Bill Rosenberg says it would rule out a proposal by the Labour Party to introduce stronger rules for foreign investment.