Child sex offenders are among criminals who have been deported to New Zealand from Australia - with 19 arriving in a year.
Those deportees arrived before the Government rushed through a new law last November to enforce parole-like monitoring.
At least 1000 deportees from Australia are expected over five years after a hardline law change in late 2014 saw a surge in people being kicked out of Australia for criminal histories or on character grounds.
Police have conservatively estimated the cost of crimes that will be committed by deportees over the next six years at $126.5 million - although Police Minister Judith Collins says reoffending has not been nearly as bad as projected.
Crimes already committed include sexual offending against a child and armed robbery, documents obtained by the Herald under the Official Information Act reveal.
In a recent case, a deportee left Australia on a charter flight with seven security staff after Qantas refused to take him on a commercial flight. Fewer than 10 days after arriving in New Zealand, he had committed an aggravated assault and robbery with another deportee from Australia.
Sent back to New Zealand
Between October 2014 and September 2015 a total of 173 deportees arrived in New Zealand from Australia, including:
• 5 convicted murderers.
• 26 convicted sexual offenders - 19 of them being child sex offenders.
In another incident, a vehicle was pulled over after running a red light and almost colliding with a police car. All four occupants were recent deportees from Australia.
"Information suggests a number of deportees from Australia have been connecting and socialising together ... it appears that in the absence of family and support in New Zealand, some deportees have been seeking other deportees in a similar predicament. This is likely to raise the risk of deportees offending in groups," says a police intelligence report from November 17.
From January last year to April 22, 385 deportees had returned to New Zealand, of whom 55 had been involved in 104 offences. Twenty-one have been involved in violent offences, and one a sexual offence.
A police document from September states that serious offences have included sexual offending against a child.
In an effort to understand how deportees might reoffend in the longer-term, police intelligence examined a cohort of 47 deportees who arrived from Australia between October 2013 and September 2014.
Twenty-six of them went on to commit 243 offences in all - an average of 9.3 offences each. Offences included assault, kidnapping, robbery, and sexual assault.
Australia amended its migration act in late 2014, leading to more New Zealanders being kicked out.
Police also looked at a second cohort of 173 deportees who arrived between October 2014 and September last year, and found 20 per cent have since offended in New Zealand.
"Given that the majority of the 2014/15 cohort of deportees has not yet been back in New Zealand for 200 days, it can be inferred that for most of these deportees, their offending is yet to commence or become evident," says the November police intelligence report.
"Brief analysis of the offending history of the 2014/15 cohort of deportees indicates an even more serious nature of offending ... for example, the recent cohort contains five convicted murderers, whereas the 2013/14 cohort had none. Also the recent cohort contains 26 convicted sexual offenders - 19 of them being child sex offenders."
Ms Collins said the projected costs of reoffending were estimated before significant improvements were made in New Zealand.
"Offending that has come to police's attention following the law change has been lower than projected, and it is positive to see that the efforts of agencies may be reducing this impact," she said.
Only 10 of the 188 deportees who returned since the Government changed the law to allow parole-like monitoring conditions in November had offended, Ms Collins said.