Malachi's Avanti Black Thunder bike was taken from the porch of the family home in Nawton some time over the weekend.
Ms Madden put posts online at Trade Me and Facebook looking for help to get it back.
"It is a custom-made bike by Trikes New Zealand in Levin. You can't just go and get one of these bikes from the shop. It took nearly three months for them to make it for us, and it cost close to $2500 to build it.
"It's so obvious it's for a person with a disability so why would you want to target this person? It's not like we can go to the shop and pick another bike up."
Malachi, who did not start walking until well after his second birthday and began talking only at age 6, has a long list of health problems since he was born 16 weeks prematurely weighing only 695g.
At 8 weeks old he contracted meningococcal disease, which caused brain damage and nearly killed him.
He also developed a chronic lung condition which makes it difficult for him to exert himself.
Malachi has had an operation to help correct a condition which makes him tip-toe when he walks.
He is to have further surgery in May to help this.
Ms Madden said she would now have to drive him to school. He could not ride other types of bike.
"If he walks to school he is absolutely spent for the rest of the day and can't do anything else.
"He gets quite exhausted and can't manage anything else."
Ms Madden said Malachi had the bike for about a year.
He received it through the help of CCS Disability Action, which approached a trust in Te Puke which provided the funding for it.
Ms Madden had thought that Malachi would eventually outgrow the bike and another child with similar needs would be able to use it.
"So I'm not quite sure what happens in the event of it being stolen. I think it's every five to 10 years you are eligible for it."
"I feel stink knowing there are other people who also need the funding. I guess that's why I'm pushing this so hard so hopefully it will surface somewhere."
Waikato police spokesman manager Andrew McAlley said Ms Madden had laid a complaint about the theft.
"We would appreciate any information that would help us return the trike to its owner, and police, like the rest of the community, are concerned offenders would target such a vulnerable victim," he said.
Ms Madden's Trade Me post was de-listed last night because listings have to be for goods for sale or services.
But a support person said the website had had success in helping have stolen goods returned.