He said he changed his name three years ago but was born as William Ringo Ratapu-Howard in South Auckland, where he still lives.
He said the Government had confiscated his passport last year, stopped him travelling to Sydney in May where he planned to marry his Lebanese fiancée and that he had repeatedly been questioned by the SIS about his beliefs.
Mr SaifuAllah said he converted to Islam about three years ago to escape his life of drugs and crime.
"I was in Black Power. I couldn't see my life going anywhere, I was having a bad life, I was a gang member."
His older brother found the religion first, and introduced it to their mother and then him, he said.
The religion has led him away from those aspects of his past. However, he has also been introduced to radical beliefs that see him sympathise with the terror group.
"No I'm not against [Islamic State], for a myriad of reasons. Am I considered a member, like a member of Isis? Well I support them of course because I support what they do.
"I stand by my views 100 per cent."
He blames the SIS for his beliefs, he said.
"I never used to support Isis or anything, I used to think they were just a pack of people that used to run around and shoot people. But the SIS have put so much into my head, they just kept asking me about this group and made me look into it even more and these people are to blame for some of the views that I have."
He said he did not believe the image of IS that was portrayed in the media and preferred to get his information from Facebook, Twitter and chat forums.
Mr SaifuAllah who has also used the surname Curtis and says he is actor Cliff Curtis' nephew, said there is a network of other young men in New Zealand who hold the same beliefs as him.
"It's quite rare to come across someone that has the same methodologies as me, but there is actually a few brothers out here that have it.
"People are waking up...it's not like getting together in one big terrorist hub or anything but people are just understanding the reality."
He does not want to leave New Zealand to fight, but believes others should be able to if they wanted.
"[The SIS] are the ones radicalising [people like me], if they want to leave - let them leave...and you can get them in Syria, you can bomb them over there but don't leave a person in the country where you know they are going to do something wrong."
Mr SaifuAllah said he did not believe a terror attack in New Zealand would be beneficial for IS.
"It wouldn't bring any benefit here, it just makes things harder for them. But if there was a place where there was a lot of Muslims...I don't see a problem with it.
"There's a reason these countries are being hit it's because they are full on in the campaign against then."
Alarming Facebook content
Mr SaifuAllah has multiple photos and comments on his Facebook page professing allegiance to the terrorist group.
In a 2014 interview with TVNZ, he said he did not support the views of the terror organisation. He does now.
"These are my views and I support them 100 percent," he said.
His photos are captioned with war cries and comments made this week spoke of his role in a holy war fought so "one day Islam will dominate everything that the sun sets on".
He has images of the Islamic State flag and other propaganda from the terrorist group.
Photos and graphics show soldiers, war cries and religious quotes.
Comments in recent days show his allegiance to these sentiments. "One day Islam will dominate everything that the sun sets on meaning the whole earth," he said on Wednesday.
Another post says: "We may lose a village here and there but always have the yaqeen [certainty] that we will always be victorious and Allah says in the Quran that Allah will always make the believers triumph over the disbelievers."
A photo posted last month shows him holding what appears to be an AK47-style rifle.
One commenter posted beneath the photo: "Are you expecting a visit?"
Mr SaifuAllah replied: "Hopefully"
"Soon they will come. Tell them about Islam and hellfire," was the response.
In the "about" section on his Facebook page, Mr SaifuAllah has written: "Do not be left behind in this Dunyah [world], work for your akhira [afterlife] not this life, it's only a temp life here."
An extremist threat?
When contacted by the Herald, a police spokesman said: "We acknowledge your query but as you can appreciate, NZ Police does not comment on security matters."
"Police and other agencies are aware of and have an interest in the activities of people who might hold or express views that could be of security concern, and will take appropriate action where required.
"We do not comment on individual cases."
Intelligence expert Dr Paul Buchanan said the man appeared to be "unstable".
"This guy looks to be a Maori or Pacific Islander who has converted to Islam and is somewhat unstable," he said.
Dr Buchanan, a former terrorist profiler for the CIA, said that making threats of violence - especially the comments about the gun - was a crime.
"Obviously the cops need to take notice of him. If he's sitting there with a gun saying 'hopefully' he gets a visit, I assume the Armed Offenders Squad are going to turn up."
He said the page was the exact sort of activity New Zealand authorities should be monitoring.
Dr Buchanan said the perceived threat of such blatant pro-Isis sentiment - as seen on this man's Facebook page - has changed.
Previously, someone who made comments openly on public forums would be considered a "blow hard" and not a serious threat.
However, that was no longer the case.
"Things are slightly different now thanks to the very adept use of social media by the likes of Islamic State. Young men who are alienated and angry and who come from a Muslim community may see his Facebook page and act upon it.
"This guy may be all bluster, but there may be someone reading his page who may be motivated to act upon his exertions."
Security Intelligence Service director Rebecca Kitteridge has previously said Facebook pages encouraging violence on behalf of Isis are among factors increasing New Zealand's official terrorism risk.
The SIS maintains a watch list of between 30 and 40 people who are considered to be a risk and with links to some form of extremism.
The Herald this week revealed violent extremism as one of the six main threats to New Zealand.
