Special votes could see the Greens gain a seventh MP but the prospect of National overturning Labour's slender 22,751-strong party vote lead seems remote.
The Greens are the most likely to benefit from the counting of the specials, the results of which will not be known until Saturday, October 1.
Jim Anderton's Progressive Party may also gain another seat on specials - it now has only Mr Anderton coming back - but that is being tipped as an outside chance.
There are an estimated 218,000 special votes to be counted, 9.6 per cent of the total votes cast.
More than 200,000 special votes were cast in each of the previous two elections. The Greens were the beneficiaries on both occasions, picking up an extra MP each time.
Greens co-leader Rod Donald said the party's election-night share of the party vote was 5.07 per cent and it needed to lift that to 5.32 per cent to get sitting MP Nandor Tanczos back.
If that happened National would lose a seat from its allocation, reducing it to 48 MPs.
Labour's hand in forming a government would be strengthened as the Labour-Green-Progressive combination would be 58 seats in the 122-seat Parliament.
If the Greens fall below the 5 per cent threshold that would pose major difficulties for Labour in trying to form a government.
Special votes boosted the Greens party-vote share from 6.49 to 7 per cent in 2002 and from 4.9 to 5.16 per cent in 1999.
Labour was at 41.36 per cent on the night in 2002, falling to 41.26 per cent on the final count. National fell from 21.08 per cent to 20.93 per cent.
On election-night results the Progressives' second sitting MP, Matt Robson, has missed out but lifting its vote only slightly from 1.21 per cent would see him back.
Mr Robson said he had not calculated the figure, but Mr Donald said the Progressives needed 1.23 per cent.
If he is returned and the Greens gain a seat as well, it would be Labour's turn to lose a seat.
