“The law used to let people who started a case stop it if they wanted to, but this rule doesn't exist anymore.”
This section of the law has been removed. It used to be called ‘Prosecution may withdraw in certain cases’ and was part of the rules about trials and sentences in the Crimes Act 1961. The government took it out of the law on 1 July 2013, so it doesn’t apply anymore. If you need to know about how prosecutors can withdraw from cases now, you should look at the current laws.