The MP who drafted the law change that banned smacking says it will be difficult to draw conclusions from tonight's referendum result.
The postal ballot closes today and the result will be announced about 8.30pm.
The 2007 change to the Crimes Act made it illegal for parents to use force against children for correction but also allowed police the discretion not to prosecute inconsequential cases.
Opponents forced the referendum, which asked: "Should a smack as part of good parental correction be a criminal offence in New Zealand?"
They are anticipating an overwhelming "No" vote and have set up a press conference tonight to claim victory.
Green Party MP Sue Bradford said today she expected a high number of spoiled ballot papers had been sent in.
"Feedback I've received from the public over the last few weeks tells me a lot of people feel pretty angry at the confused nature of the referendum question and the waste of $9 million of taxpayers' money this represents," she said.
"For some, their response has been to write political - if not downright rude - remarks on their ballot papers."
Ms Bradford said the question was so flawed with its mixed and ambiguous message that she knew of strong supporters of the change who had voted "No".
"I would have a much greater respect for the referendum result if it was based on a clearer question," she said.
"The `Yes' vote is a vote for keeping the law as it is, providing children with the same legal protection from violence as adults.
"Even a large `No' vote tonight won't be a clear mandate to the Government to act in any particular way," she said.
The referendum result is not binding on the Government.
Prime Minister John Key has said the law is working in the way that it was intended to, and he would not change it unless it stopped working.
NZPA