At the Court of Appeal Swain claimed his lawyer was incompetent and failed to follow up leads which included a witness who said she had heard rumours that someone else was responsible for Hansen's death, RNZ reported.
He also said the judge had failed to give the jury appropriate direction, and interruptions during video evidence had disrupted cross-examination of a key witness.
RNZ reported Justice Venning's decision released recently found the technical problems were unfortunate but would not have led to a miscarriage of justice.
He said Swain signed off on his defence and the judge's summing-up did repeat the points made by the defence about the unreliability of one witness's evidence.
Swain's appeal against his conviction and sentence were both thrown out.
Whetu Hansen's body has never been found.
At the 2015 sentencing, Justice Brendan Brown said despite no clear motive being established Swain had performed a "deliberate killing using a firearm on an unarmed man".
"You are a danger to the community ... you are a hardened criminal, not deterred by a long term of imprisonment."
As Swain was escorted from the court in 2015 he said: "For once I am innocent but found guilty".
"I am disturbed you have allowed police to lie and perjure evidence ... I am very, very saddened by this. How can I tell anybody where a body is if I don't know where it is."
