There aren't enough police to monitor sex offenders if they are put on a register, Labour says.
Police Minister Anne Tolley wants a register and hopes to have one in place by 2014.
She says that at the end of their sentences sex offenders disappear into communities and authorities lose track of them.
Labour's police spokesman, Kris Faafoi, says budget cuts are putting police are under increasing pressure and they're being told to do more with less.
"It's one thing to have a register but it's another to have the resources to monitor them," he said on Tuesday.
"I'm sure front line officers will be asking `who is going to do that?'." The Government first signalled it was looking at a register in April, after a convicted child sex offender was found to have worked in several schools.
At the weekend Mrs Tolley, who recently visited Europe and discussed similar registers there, said she would take a proposal to cabinet next year.
The proposed register would be accessible only to Government departments and Labour is also concerned about potential privacy breaches.
Corrections spokesman Charles Chauvel says repeated breaches at Government agencies, including ACC, Inland Revenue and Work and Income, highlight the risks.
"If you have a look at this Government's record of not being able to protect personal information, I think you start to see the problems with this proposal," he told NZ Newswire.
Mr Chauvel says the Government needs to show that it can manage data properly, and what impact the register might have on offenders' rehabilitation, before it's allowed to proceed.
"Otherwise, who knows where this information will end up? And if it's wrong in any respect then the consequences for individuals will be catastrophic."