Sources report that Labour's Carmel Sepuloni is ahead by fewer than 10 votes after the counting of special votes.
Ms Bennett will remain in Parliament as a National list MP if she does lose Waitakere, but it will be embarrassing for the "proud Westie'' who surprised many by winning the previously safe Labour seat in 2008.
Such a close result would almost inevitably result in a judicial recount, which could take until Christmas.
A recount is also likely to be sought in Christchurch Central, where Labour's Brendon Burns and National's Nicky Wagner were in a dead heat on election night and are understood to be within 100 votes of each other after the counting of 3717 special votes.
It was not known last night who was leading.
Final election results are to be released by the Electoral Commission tomorrow afternoon, and candidates have three days to apply for a recount.
On election night, Ms Bennett was ahead in Waitakere by 349 votes.
But 3413 special votes were cast, and sources said scrutineers watching the counting were reporting Ms Sepuloni had inched ahead.
Yesterday, Ms Sepuloni said she was waiting for the official phone call to say whether she had won or lost before she believed anything.
She must win the seat to get back into Parliament.
Ms Bennett said she had heard the count was "very close''.
She said she would expect to seek a recount if she lost by a small margin, but had to discuss it with her party.
"I think we are in for a judicial recount either way. If it is within 10 or 15 votes, it would be crazy not to.''
Ms Bennett said Ms Sepuloni had concentrated hard on winning the seat and mobilising the vote on the left.
"We always thought there would be a scrap out West and so it has turned out to be.''
In Christchurch Central,
Ms Wagner said she expected a recount regardless of who won.
"It [the cost] is $1000 or something like that ... Certainly I'm sure both Brendon [Burns] and I would be happy to pay $1000 just to find out for sure.''
Mr Burns would not comment yesterday.
The Green Party could also gain an extra seat from the special votes.
It would go to Mojo Mathers, who would be New Zealand's first profoundly deaf MP.
Special votes are cast by people outside their own electorate on polling day and others such as hospital patients and overseas voters.
If Ms Sepuloni and Mr Burns win their electorate seats, list MPs Raymond Huo and Rajen Prasad will not get back into Parliament. If only one of the two wins Mr Huo will be out.
- Claire Trevett and Derek Cheng, The New Zealand Herald
