He was not subject to the ESO at the time of the murder of Blessie Gotingco because he was subject to similarly restrictive release conditions set by the Parole Board when he was released in December 2013.
The ESO was sought by the Department of Corrections and due to come into force for 10 years from June 2014.
Figures obtained through the Official Information Act show the the Department of Corrections obtained 140 Electronic Supervision Orders until August last year. The figure would have included the order granted against Robertson in March 2014.
The total was reached after 10 years of the law allowing ESOs being in force. It was passed by Parliament in 2004 and peaked at 242 child sex offenders, of which 26 were in custody in November 2014 for breaching the order or fresh offences.
The average cost of monitoring an offender subject to an ESO is $27,000 a year although costs vary. It stated those under the most stringent Parole Board conditions could cost $300,000 a year.
The figures show a drop from 38 people given an ESO in 2009-10 to 18 in 2013-14, which Corrections says is due to the "finite" number of those in the community who qualify.
ESOs were considered for those who showed the highest level of potential risk, and always for those who refused to accept any treatment.
Victoria University Professor Tony Ward - formerly Director of the Kia Marama Sexual Offenders' Unit at Rolleston Prison - said sex offenders reoffended at a "significantly
lower rate" than other people released from prison.
He said those under the ESO regime needed support on release to avoid isolation - a prime factor in re-offending.
"The problem with demonisation is it isolates the person from the community. The best way to keep us safe is to keep an eye on someone."
Support included making therapy available, encouraging employment opportunity and involvement in community groups. He said the closeness of the community around a released sex offender made it more difficult for that person to offend.
"The public understanding of sexual offending is so poor that they think people are walking around with this destructive potential for sexual offending that's just seconds away."
The University of Auckland's Dr Gwenda Willis said media skewed perceptions of sex offenders and gave people an unrealistic fear of stranger-danger.
"The cases do happen but they are rare. People around you are probably a greater risk than someone who has been in prison."
She said the 140 people who were subject to ESO orders gave no insight into those in society who had not been caught - and most child sex crimes were committed by people known to victims.
Dr Willis said risk scales were not absolute measures. "There's no way we can say 'this person is (a certain) per cent likely to reoffend'. It would be nice if we could predict but it's not that straight-forward." She said the measures was relative to others who were committing the same sort of offending.
Excerpts from the High Court ruling that granted an Extended Supervision Order:
"There is a real and ongoing risk that cannot sensibly be ignored, having regard to the nature and gravity of the likely offending." - Justice Edwin Wylie
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"It is also clear that if he is to reoffend, the harm that would be caused to his victims would be very serious indeed." - Justice Edwin Wylie
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Psychologist Kirsten Tolond said Robertson had problems with a "limited capacity for relationship stability, his hostility towards women, his sense of general social rejection, his lack of concern for others, his impulsivity, his poor problem-solving skills, his negative emotionality, his sex drive and sexual preoccupation, his deviant sexual preferences, and his lack of cooperation with supervision".
She considered that his sexual reoffending was "likely to place female children who are unknown to him, including those in public places, at risk of abduction, indecent assault, or further sexual offences".
