Immigration New Zealand say it should have required a character waiver before granting residency to a registered American sex offender.
Guy Verschuur and two female trampers were rescued from Kahurangi National Park at the top of the Sourt Island on 22 April after they failed to return from a three-day tramp that started on 15 April.
At that time, it was discovered that the 51-year-old was listed on a sex offenders' registry in Nebraksa.
Immigration New Zealand said last week it would review Mr Verschuur's file after his conviction became public.
On Tuesday, general manager Nicola Hogg said the man did declare his conviction when he applied for residency in 2008.
She said the case officer decided that in spite of the conviction, Mr Verschuur met character requirements.
Ms Hogg says in hindsight, a character waiver should have been required because Mr Verschuur had been sentenced to prison.
Immigration New Zealand is investigating whether further action is required.