Community liaison nurse Grahame Stillwell said the young man was taken into custody on Sunday, after allegedly assaulting his mother.
He was assessed by the Mid-Central District Health Board crisis team, who said they were unable to perform a full assessment due to the man's challenging behaviour.
However, the MidCentral District Health Board said that report was incorrect.
The DHB's mental health services' clinical director, Syed Ahmer, said: "MidCentral is aware of the needs of this young man and is aware of assessment and treatment options for him that do not involve placement in a mental health facility.
"As a general principle, we can confirm that our services are always available to people who present with current mental health treatment needs, including completion of thorough clinical assessments.''
Ahmer also said: "MidCentral is endeavouring to make these options known to the authorities to ensure the safe and appropriate care for a young man with reportedly severe intellectual disability and autism.''
Stillwell said nothing had changed since then, and no agencies had responded to his emails asking for help.
The young man was in a prison T-shirt and is being held in Whanganui Prison in the at-risk unit.
Outside the court today, the mother said her son's violent behaviour had always been an issue but this had worsened in the last two years.
"I was getting some support from IDEA Services and Enable who does the funding side ... but due to his challenging behaviour they have reduced any support to minimal.
"I was saying more, more, more and they are saying less, less, less."
Respite care was reduced to once a month, his school holiday programme was reduced to three days a week, for two hours and his after-school care was withdrawn altogether.
She said she was not consulted and given no other options for alternative care.
Speaking of Sunday's altercation, she said she felt compelled to call police as this was her son's only chance of getting help.
"I was told it was the right thing to do because it would flag him in the system. But, it shouldn't have got this far.
"If the right people had done their job, we wouldn't be here."
She said she felt let down by IDEA Services and had lost confidence in disability service providers. She said they provided emergency accommodation and couldn't understand why this was not offered to her son.
"They have the option but didn't offer ... We just want to find a placement for him; someone that will help and do what's best for him. He can't come home with me. I can't cope ... Sunday could have ended up with me really hurting my son because, this time, I had to physically push him off me."
She said her son was troubled but he also had a special connection to his family.
"When he's good ... he's cheeky, smart, butter wouldn't melt ... he's got the whole family wrapped around his finger."
During his court appearance, the young man was whispering and repeatedly asking if he was going home to his mum.
"He is 18 but he doesn't have the brain capacity or the mental capacity of an 18-year-old.
"He probably doesn't even know he is in a prison."
Grahame Stillwell told the court this morning he phoned Mid-Central District Health Board on Monday to ask if they had a plan in place, but they did not.
He said since then, the man has been held in custody and the prison guards had been doing a great job - as were police - but the agencies who are responsible had ignored his emails.
Stillwell said he also told the providers there was media interest, in a bid to highlight the "absolutely outrageous nature" of the situation.
"It was just crazy," he said.
Judge Lynch agreed the situation was outrageous.
"We have a young man in jail because services aren't stepping up to the plate," he said.
Using the criminal justice system was not appropriate for a young man who has serious challenges, and would be unlikely to be fit to plea.
Judge Lynch said he could not keep the teen in jail. He needed somewhere to go and the criminal justice system should not be used "in the way that is being forced on me".
He said the only option may be to put the man into a hospital, and he might very well do that when the young man was due to reappear this afternoon.
In his second court appearance today, the 18-year-old was told he would be allowed out of jail and would be spending the next 10 days at a residential care facility in Levin.
After that NZ Care would accommodate the boy long-term.
Comment has been sought from IDEA Services but it is yet to respond.
