The government is set to reveal its plans on "fixing" prisoner compensation at Cabinet today. Unfortunately, the media doesn't seem quite so sure on what exactly is being revealed. According to Stuff, the government will reintroduce the right to sue, at least for victims of criminals who have received compensation. The Herald, on the other hand, thinks that it will involve giving judges discretion to award some or all of any compensation paid to victims. I guess we'll either have to wait and see or watch the Knowledge Basket bills archive for the details.
Something both media outlets agree on is that the rules change will apply from the date the bill is introduced into Parliament. But unless it is passed on the same day, this will be retrospective legislation, no matter how they fudge it. While it's not illegal to pass retrospective legislation, it is generally unconstitutional (tending as it does to breach natural justice), and until the bill is actually passed, it cannot be used to prevent claims being filed or cases being progressed.