There are growing calls for an overhaul of the prosecution system after three high profile acquittals in the past week.

New Zealand is one of the last countries in the Western world to employ private lawyers to prosecute police cases and defence lawyers and the Green Party are now calling for the establishment of a public prosecution office.

The office would be independent of the police, which would bring New Zealand into line with the US, Australia and the UK.

Lawyer Chris Comeskey said the not guilty verdicts in the recent cases of Chris Kahui, George Gwaze and Murray Foreman have come from juries sitting in courts across New Zealand and are not a coincidence.

The police have defended their investigations in each case. In the Kahui and Foreman cases, they have released statements to the media which said while being disappointed with the verdicts, they respect the decision of the jury.

But Mr Comeskey said: "There is something wrong with the way things are investigated. The police are rushing off with too little, too soon and ending up with nothing as a result.

"The second difficulty which flows on from that is you have [legal] firms scattered throughout the country which hold Crown Solicitor warrants and that's not done anywhere else in the world and for probably good reasons."

He said state-run prosecution offices in other countries maintain an independence from police because they are not a client of the police.

"Under the set-up we have, the police are the client of these law firms and whatever the police ask for, they are granted," Mr Comeskey said.

He said an independent prosecution service could provide another check for police prosecution cases and may ask for more investigation into a certain lead or another search warrant, for example.

Green Party justice spokesman Nandor Tanczos said: "The fact that people are acquitted does not mean the system is failing. However these cases raise three questions. Did the police charge the wrong people? Did they ignore alternative explanations? Did they simply not have enough evidence to convict?

"The establishment of a public prosecutor's office would help avoid all of those issues."

Criminal Bar Association president Graeme Newell believes there is also a serious problem with experience in the police force.

"I wonder if we are retaining the level of expertise that we once had because police officers leave to pursue other careers much more readily than they used to. It's not uncommon to see junior officers rising through the ranks," Mr Newell said.

He said police sometimes focus on only one suspect, particularly in sexual abuse cases.

"It is common in these cases for there to be less investigation then we would expect. I think juries are often surprised by the lack of investigation," Mr Newell said.

Defence lawyer Barry Hart said the three acquittals could indicate that jury members are "better educated" after watching television dramas and documentaries that detail what happens when an innocent person is locked up.

He said added to this is a relatively new belief that the police don't always get things right and that there are some "bad apples" in the ranks.

Mr Hart said there is a need for a separate public prosecution office which would provide more independence.

"The police and the Crown Solicitor's office are too close," he said.

Solicitor General David Collins QC said the "cluster of verdicts" in five high profile cases being released in a short timeframe is unusual but all of the cases are unique.

Besides the Kahui, Gwaze and Foreman acquittals, there were the two cases where defendants Ian Crutchley and Lipine Sila were found guilty.

Mr Collins re-iterated that he was considering an appeal to the High Court on the Gwaze case.

"The police and Crown will conduct assessments of their respective roles in all five cases with a view to ascertaining if anything can be learnt from these experiences," Mr Collins said.