An Interpol check revealed he was convicted of importing 4.5kg of cannabis into the United Kingdom and was sentenced to two years in prison in March 1999.
While his identity was bona fide, Ms Fergusson explains Onovo needed the forged hospital letters to mask the time in an English jail. "So you're never going to find that out at the border, or by a case officer."
Despite the immigration fraud, she believes Onovo was indeed a qualified psychiatrist - a belief shared by the Canterbury DHB.
A board official said his credentials included a certificate of good standing from the Irish Medical Council that showed at least three years of good practice. Referee checks with doctors who worked with Onovo were also made.
The 42-year-old Onovo, who is married with children in Ireland, was sentenced to 16 months in prison in October 2010. A few weeks later he was followed by Hakeem Amoo Ewebiyi who was jailed for two years and two months for immigration and passport fraud. Ewebiyi, who later worked as a mental health worker to rehabilitate sex offenders in Christchurch, came to New Zealand under a false South African passport with a different name in 2003.
His bid for refugee status in his real name was declined but he was granted a work permit. The 43-year-old later gave Immigration officials two different birthdates - January 1, 1969 and January 1, 1975 - when applying for residence but his Nigerian passport was also discovered to be fake, with pages missing.
He also claimed to have lost the false South African passport, which police later found in his home with a false Nigerian driver's licence and identity card from Congo.
The deceptions were almost identical for Auckland engineer Jeffrey Ugochukwu Orji. He came here in December 2002 with a false South African passport under an assumed name.
His refugee claim under his real name was declined and he applied for residency under a Nigerian passport. However, close examination revealed the document had been tampered with and the true serial number came from a batch of stolen passports. Orji was sentenced to two years and two months in prison in February.
The three apparently separate cases of immigration fraud would never have come to light if not for Operation Naira - code-named after the Nigerian currency. The wide-ranging inquiry was launched in 2009 after intelligence from officials in London suggested immigration applications from the West Africans were "high risk", particularly around taking advantage of New Zealand's secondary migration policy.
Fraudulent secondary migration is where a principal applicant successfully acquires permanent residence, by false pretences, and as a result helps other family members to come here as residents - and be able to receive taxpayer-funded benefits such as health care, education and social welfare.
Operation Naira reviewed 160 cases, led to 43 permanent residency applications being declined and charges laid against eight people.
The inquiry continues, with three cases before the courts, and Kerri Fergusson is reviewing other cases where citizenship or residency has been granted.
Immigration fraud is damaging for two reasons, according to a Department of Labour report in 2010.
Fraudulent claims displace or delay applications and claims by genuine applicants, which is particularly damaging for asylum candidates by genuine applicants.
"Secondly, once individuals obtain New Zealand residency - and potentially citizenship - through fraud, it is difficult, time-consuming and expensive to fix this."
Each refugee fraud costs the Department of Labour an average of $28,550. This does not include "downstream" costs to taxpayers of legal aid, health care, education, state housing, welfare, police, courts and custody costs.
While Operation Naira was successful in stopping 43 fraudulent residency applications through secondary migration, the sweeping inquiry also illustrated the immigration problems that authorities faced.
The investigation came after an Auditor-General report in 2007 criticised the Department of Labour's management of identity fraud detection. Kevin Brady noted several areas where improvements needed to be made and made 15 recommendations, including that the department "regularly identify immigration identity risks specific to the skilled migrant and UN-quota refugee entry categories".
Greater training and guidance for frontline staff involved in detecting fraud, such as customised service officers and refugee quota immigration officers, was also needed.
"Generic training is provided for these staff, but there is little training that is specific to the detection role," Mr Brady wrote.
The criticism was taken on board. Irregularities in the applications from Orji and Ewebiyi were noticed by diligent case officers and referred to Ms Fergusson for further investigation.
The Auditor-General also noted the absence of routine-use of biometrics to verify identity, technology which is now being introduced.
Peter Elms, head of fraud for Immigration New Zealand, said countries such as Nigeria and Somalia posed some difficult challenges as the region is too dangerous to have New Zealand officials based there.
The quality of legitimate passports from those countries was variable and it was hard to access a central record to "find out who has been issued with what documentation".
Endemic corruption meant that even if a passport was legitimate, Mr Elms said, it may not have been given to the right person.
Then add in the difficulties of checking the bona fides of asylum seekers, whom UN convention dictates can often flee their countries under false names to escape persecution.
But Mr Elms said border control has tightened considerably since Hakeem Ewebiyi and Jeffrey Orji came here on false passports.
Upgraded passport scanners now guard the border, with greater technology to sniff out a forgery. And every single person flying to New Zealand is pre-screened overseas before they board the plane. This has led to asylum claims dropping to around 300 each year, down from a peak of around 2000.
New Zealand now works more closely with international partners, particularly the "Five Country Conference" of the United States, Canada, the United Kingdom and Australia.
Recent changes to the Immigration Act mean the levels of co-operation will increase as New Zealand can now collect and store biometric information, such as fingerprints and photographs.
"The reality is the capturing of biometric data is the single most effective tool in countering identity fraud," said Mr Elms.
"If someone is coming here, eventually they'll have their faces and fingerprints captured. That may be in the wrong name, or a false identity. Next time they come in under another identity, the falsehood will be detected."
Moves are also underway to have access to Interpol records.
Mr Elms said that despite the technology "we never underestimate good nous and investigative skills to recognise when something doesn't add up".
New Zealand's secondary migration policy ripe for exploitation
The modus operandi used in Operation Naira isn't limited to Nigerian scammers trying to start a new life in New Zealand.
Lead investigator Kerri Fergusson noticed a similar trend in other African nations, particularly in exploiting this country's secondary migration policy. Department of Labour figures show 20 Somalis have been prosecuted for identity fraud since 2004.
Sahra Ismail and Ali Abdi came here in 2002 as Somali refugees and were later granted New Zealand citizenship. But the pair travelled here on false names and documents, as they were already refugees and citizens of Holland.
They had lived there for up to 12 years but efforts to have Ismail's family join her were thwarted by Dutch authorities.
"They thereby duped this country in crossing its borders to finally become citizens of New Zealand in that way," Judge Philip Connell said in sentencing the pair. "She had a knowledge of New Zealand's secondary migration policy and that in part was probably one of the reasons she chose New Zealand to carry out this fraud."
Exploiting the policy, Ismail successfully applied in 2004 to have seven family members join her as refugees - including her sister Qamar Aden and brother-in-law Abdirahman. In turn, Aden arranged for another four people to come here.
Judge Connell said the immigration fraud committed by Ismail, Abdi, Ali and Aden was "premeditated and persistent".
He sentenced Ismail to two years nine months' prison in April 2011, while Abdi's sentence was reduced on appeal to two years and six months'.
Aden and Ali were both given six months' home detention. The Weekend Herald can reveal that Sahra Ismail and Ali Abdi have also been convicted of benefit fraud receiving about $64,000 more than they should have.
Ismail was sentenced to a further four months in prison and Abdi to one month last March.
