Kurisaqila told the Sunday Star-Times he was "struggling" to find a motive for their brutal murders in July. Apete Kaisau, 23, has been charged with the killings.
This comes despite well-publicised comments from Fiji police that Kaisau killed the couple because he had been sexually exploited by them.
"We have no proof that there was a relationship like what was alluded to by police, like a homosexual relationship," said Kurisaqila.
"(The motive) really stumped us from the beginning. There's a lot of talk but at the moment we're struggling with that one.
"I don't think it's political but as far as sexually motivated, there's nothing that indicates that it is."
Kaisau is in custody and awaiting a preliminary hearing. His New Zealand lawyer Barry Hart this month applied to the Suva court for a psychiatric report on his client.
Kurisaqila said the request might indicate an insanity defence but Hart refused to comment, other than to say "many facets" would emerge at the trial.
According to Fiji law, if the accused was found mentally unfit, the court would request he be sent to a mental institution and the trial aborted, said Kurisaqila.
He said Kaisau had admitted the killings to police and did know the pair. Kurisaqila said the murder was not politically motivated as suggested by Scrivener's New Zealand family after the deaths.
Drug and paedophilia claims by police against the murdered couple have been rife since they were hacked to death with a cane knife in their Suva home on July 1. Associates and young Suva boys who knew the pair have alluded to sexual liaisons with the couple but Scrivener's family denied the claims.
Preliminary tests show white powder found in the couple's home was cocaine. Police are awaiting further forensic tests being carried out in Australia.
Kurisaqila said public speculation would make the trial difficult.
"I really don't speak for (the police) but it really has made our job a lot harder because right now with all of this in the public domain, trying to get impartiality on this case is going to be impossible."
Scrivener's sister, Janice Giles, commended the call for a psychiatric report. The family believed Kaisau was delusional.
"My family's position hasn't changed, we don't believe any of the allegations," said Giles.
Hart said a preliminary hearing could not be held until he'd received all documents from the prosecution. He maintained the motive was not political but would not comment on the defence case or Kurisaqila's comments.
He agreed public speculation about the murders could affect the trial. "There's hardly anyone who hasn't heard of the case . . . it won't be an easy case and could go both ways," said Hart.
A 60 Minutes show tonight claims the couple may have brought death upon themselves. It has interviewed young Fijian men who claim Scott and Scrivener plied them and Kaisau with drink and drugs and abused them.