Alleged offenders of sexual crimes now have more chance of walking free than being convicted.
Answers to parliamentary questions from National's police spokesman Chester Borrows show the proportion of sexual offending allegations resulting in a conviction dropped from 55.6 per cent in 1999 to 44.6 per cent lastyear.
Most of the drop occurred over the past three years with the biggest drop - from 54.8 per cent to 46.7 per cent - occurring between 2003 and 2004.
Notes accompanying the figures say the system used to log cases was updated in 2004 - the year in which the largest decline is obvious.
"Accordingly, extreme caution should be used when making inferences based on any change between 2003 and 2004."
A letter from Justice Minister Mark Burton accompanying the figures also points out that cases in the Youth Court do not result in a conviction.
But Mr Borrows said the figures, which cover rape, unlawful sexual connection, attempted sexual violation and indecent assault, were concerning.
"That's a very disturbing decline, which becomes an issue of confidence in the system when victims do not come forward because the chance of a conviction is less than half."
The law may need to be changed to make it easier to convict alleged sex offenders.
"Labour needs to move urgently to restore that confidence so women do come forward. That is not happening right now."
The Government was slow to act on the 2004 Safer Communities Action Plan to Reduce Community Violence and Sexual Violence. It was supposed to establish an inter-agency group to tackle sexual violence in 2004 but had done so only this year, he said.
