Other factors included victims' wishes, admissibility of evidence, and the age of the parties at the time of the offending.
There may, however, be another reason that it ends without a prosecution - the failures of the original inquiry.
The question is this: had the three complaints to police in 2011, one from a 13-year-old, been investigated in a thorough and timely manner, would a prosecution have been more likely? Would other victims have been more forthcoming if the approach adopted by Malthus had been pursued then? Would police searches of alleged attackers' devices have produced more evidence back then?
It's hardly a surprise that, after the passage of time and welter of coverage, search warrants executed in Operation Clover "weren't overly successful", as Malthus put it.
The Independent Police Conduct Authority (IPCA) report into police handling of the original case has not yet been released. All that was made public were the IPCA's findings on a "systemic breakdown" in police communications with media.
The first and most important aspect of their inquiry - the "adequacy of the police criminal investigation and the handling of any complaints or reports received by police from members of the public between 2011 and October 2013" - was kept on ice pending the completion of Operation Clover. This report, which the IPCA says has been updated and will be released before Christmas, is hugely important.
The wider episode is extraordinary because of the public glare it has attracted and ordinary because allegations of sexual offences hardly ever result in a successful prosecution. Research suggests that for every 100 cases of sexual violence, seven are reported to police, three cases prosecuted and one results in conviction. So much for that "reasonable prospect of conviction".
Malthus told media that in the absence of a decent shot at conviction "we are not in the business of putting these young people into a court process", and who could argue with that?
In an eloquent and harrowing post at the Public Address site, a woman who has given evidence against an accused sexual offender writes: "The court process for victims of sexual assault is easily the most traumatic thing possible for someone who is already traumatised."
Leaving to one side the dysfunction that underpins a culture in which allegations of sexual assault are so pervasive, in which the predatory braggadocio of "Roast Busters" acolytes exists, the inescapable conclusion is the justice system is failing.
As Justice Minister, Simon Power tasked the Law Commission with considering alternatives to the adversarial system for sexual violence, including an inquisitorial element and restorative justice options. In 2012, his successor, Judith Collins, killed the project, saying it was incompatible with the existing system. In the lead-up to the election, however, Scott Simpson, then chairman of the justice select committee, said following Collins' resignation there was a chance for its resumption - a prospect about which he was "open and willing and keen".
With luck, that enthusiasm will rub off on the new Justice Minister, Amy Adams.
Auckland's $12b gap
The traffic is gridlocked, the solution deadlocked.
What about motorway tolls and a regional fuel levy to pay for a half-decent transport system, says Blinky Len the Mayor. Our roads, our fuel tax, hands off, says Squeaky Simon the Transport Minister.
Photo / Brett Phibbs
On this vexed matter it is time we thought outside the box, not least because it is very uncomfortable sitting in a box. Here are 10 alternative revenue models to fill that pesky $12 billion transport funding gap:
1. Hire out the Town Hall's Ngati Whatua room to burlesque dancers and swingers' parties.
2. Hire out Len Brown as a bouncy castle.
3. Offer guided tours of the Houses of Parliament from an Assistant Speaker dressed up as a moa.
4. All callers to sports talkback expressing a view on whether Jesse Ryder should return to the Black Caps to be subject to a levy of 20c per sentence.
5. Journalists to be fined $20 every time they report that someone "took to Twitter" to say something or other.
6. Require public figures to contribute $200 every time they answer a question with "What I will say" or "What I can tell you", or similar.
7. Inspirational speakers, grinning estate agents and drivers of Porsche Cayennes to be taxed $2000 daily.
8. Require the Prime Minister to contribute $20,000 every time he noisily inhales through his teeth at parliamentary question time. Also applies to leader of the National Party and Bronagh's husband.
9. The council to begin a bike recycling scheme in which cyclists' bikes are stolen from them and sold back at half their value.
10. Nationalise chocolate milk and/or modify cows' genes so they produce it directly.
