"It could open up some of the convictions that might have relied on some of these identifying factors. It could open the court process up and also mean some victims would have to go back through court."
No one had exploited the oversight in the law, she said, and the bill applies retrospectively, meaning appeals on those grounds would fail.
But Ms Collins said police had complied with the law when they realised the error and had destroyed 5500 records, disadvantaging them when dealing with repeat offenders.
She said police had been able to overcome some of these issues [but] "if we had had 10 years' worth of this, I think we would have had a real problem."
The bill was kept secret until it was introduced yesterday morning, which the Green and Maori parties described as a "travesty".
"The public didn't have any chance at all of finding out anything about this bill until we started debating it in this House," Green MP Keith Locke said.
"Any people who might be concerned about the legal issues involved in this are totally shut out of this process."
But Ms Collins said announcing the bill earlier would have been an invitation to a flood of appeals.
"The last thing we want to do is have every lawyer in the country race around and try and overturn convictions."
Police became aware of the problem in 2010, but it was not until December that it became clear that a bill was needed to fix it.
"There was a great deal of effort put in by police and Crown Law to make sure there was no extension of police powers, because then we would have had to go to select committee," Ms Collins said.
She described the error, which occurred under the previous Government, as "one of those collective failures".
LEGAL LOOPHOLE
* 5500 records of fingerprints and photographs of youth offenders have had to be destroyed.
* This is to comply with a law change in 2008 that made it illegal to keep them.
* The Policing Act 2008 meant police could keep only about 50 sets of identifying records a year.
* The previous limit was about 1200 records, which will be restored with the passing of legislation yesterday.