His New Zealand lawyer, Craig Tuck, said his legal team would be appealing the decision.
Mr Tuck said the death penalty had been "on the table" until the last moment but the significant sentence was not a relief.
He had just got off the phone with de Malmanche when he spoke to the Herald and said he was in "emotional turmoil" thinking about the reality of spending more than a decade in an Indonesian jail.
Family spokesman James Bellamy, who is in Bali, told TV3's Paul Henry that de Malmanche was perhaps guilty of being naive.
"But being naive isn't a crime. [There is] no doubt in our minds that he is not connected to any cartel."
The family had been hoping for an acquittal and that de Malmanche would be home in Whanganui by the end of the week, he said. "Fifteen years is not a firing squad, and it turns into about 10 years with various remissions, but it's not an easy lag.
"To his family obviously it's a significant blow, a fixed-term imprisonment of that length. It's a lot less than probably many others, but having said that, it's Dad away from home for a long time and especially with the vulnerability that he's got."
The defence had to tread lightly because there were dangers with appealing in an Indonesian court - a number of people have ended up on the receiving end of a death penalty after appealing a sentence, Mr Tuck said.
He said the decision had "simply sidestepped" a number of central issues. "Half the job's done and we're moving up the next level now.
"[De Malmanche] knows that everybody's still behind him. We are going to get the result that we want, it's just going to take a bit more work. But he's not off to the island to get shot, which is a big first step."
In court, Indonesian prosecutors stopped short of requesting the death penalty and instead recommended a prison sentence of 18 years.
De Malmanche's lawyers - a team of about 10 - have argued throughout the trial that he was a "trafficked person, not a trafficker".
The court heard he was abused as a child and had been institutionalised with mental illness.
De Malmanche said he was looking for love online when he met "Jessy Smith" and was offered an all-expenses-paid trip to Bali. It was the first time he had travelled overseas.
He met people who he believed were Jessy's staff in China before flying to Bali where customs found the 1.7kg of crystal meth in his bag.
Custom officials said de Malmanche was sweating so much during the security check that his jacket was "soaked wet" in sweat.
Officers found a plastic bag wrapped with tape containing the crystals inside his almost-empty bag.
The trial has lasted months, after being disrupted in April, when de Malmanche collapsed in court because of a heart complaint.
Mr Tuck said there were "a number of appeal grounds" the team was looking at and had identified throughout the trial.
"Virtually across the board in terms of procedural irregularities, behaviour of the judges [and the] prosecution, all sorts of claims made by the prosecution, lack of disclosure of certain aspects, there's just so many different points," he told Radio New Zealand this morning.
"The system is quite different to what we have in New Zealand, but having said that the rule of law applies, and they have a criminal procedure code which simply just wasn't followed from day one, and we will be appealing those points."
The prosecution had "a lot of information which would indicate that [de Malmanche] didn't have knowledge [of the drugs in his bags], which was left out of the trial deliberately", Mr Tuck said.
The legal team had "volumes of material" it could have put before the court, but the judges wanted to hear all the evidence in one afternoon, he said.
"They just wanted parts of the evidence, they didn't want all of it, and they're matters that we will be taking up with higher authorities," he told the broadcaster.
"The defence of human trafficking, they were just simply not prepared to entertain it, it seems. They continued to run this trial as they do with all other drugs trials over there on the basis that a foreigner has drugs, we've caught him, foreigner is responsible regardless of whether they are or not, and that simply is not the international legal position."
Appealing the decision would "potentially" put the death penalty back on the table, Mr Tuck said.
"But having said that there are strong grounds, there are strategy that we're working on at the moment with Indonesian lawyers to ensure the matters that came up at trial continually, week after week, can be put in front of another judicial panel one step up from the Denpasar District Court."
But it could be "some months, at least" before the appeal is heard, he said.
Timeline
December 1, 2014:
Antony de Malmanche is arrested after 1.7kg of crystal methamphetamine is found in his backpack at Denpasar International Airport.
February 17, 2015: Trial for de Malmanche set to begin.
March 31, 2015: Trial begins after more than a month of debating issues with indictment.
April 17, 2015: De Malmanche granted a week's leave from the trial after collapsing during a hearing with a heart complaint.
June 30, 2015: Guilty verdict is delivered and de Malmanche receives a 15-year jail term in Indonesia.
