The 11-year-old girl who was arrested and handcuffed by police in front of shocked classmates at her Northern Territory primary school last year has been missing for at least three weeks.

Northern Territory Police have defended their decision not to release a public plea for information until more than three weeks after they said they were first notified of her disappearance from a Darwin foster care facility.

The girl's extended family are angry. They say child protection authorities did not tell them she was missing and are accusing the department of failing its duty of care.

"What do we have to have … for it to come across the media to say 'oh they've found an 11-year-old girl killed in the scrub here in Darwin' and we don't even know she's missing?" one family member said.

"That's our girl."

Police issue notice three weeks after disappearance

The child, who cannot be identified for legal reasons, was taken by child protection in November, after she was arrested at her primary school in Tennant Creek, 875km south of Darwin, on suspicion of property offences.

The case made international headlines.

She has since run away several times from the Darwin care facility known as Yirra House.

On Wednesday, Northern Territory Police issued a concern for welfare notice, saying the last time the girl was sighted was about three weeks earlier, at the Darwin waterfront.

In a statement, police said despite the best efforts to find her, the girl had "actively avoided" them.

"Police were also aware that the girl had been in contact with the family throughout this time," the statement said.

"However, after this contact ceased in recent days, authorities became increasingly concerned for the girl's welfare, and a public call for assistance message was arranged."

Police said the girl had made contact yesterday and they organised a late-afternoon meeting, but the she did not appear as planned.

The family member said relatives only learned the child was missing when the girl called her grandmother — but that she would never say where she was.

"The Territory comes out, what, nine years ago with the Little Children Are Sacred report — if they're so sacred, what the hell are they doing? And who comes down on the departments?" the family member said.

"They're being as neglectful as the parent.

"This child hasn't been going to school, she's been running the streets and she's supposed to be in the care of the department."

The girl had been staying with her grandmother in Tennant Creek before being taken by child protection.

Extended family had wanted her to stay in their care, but said they needed help.

"We've known for a long time that [the girl] has had a few problems and all we've asked the department for is an assessment," the family member said.

"We said we wanted to full mental health assessment and also a FASD [foetal alcohol spectrum disorder] assessment … then you can find a carer that can look after her with those special needs."

The girl's grandmother told the Tennant & District Times she was heartbroken.

"I think wherever she is, she is unsafe and at high risk. Who knows what she's mixed up in and who she's with?" the grandmother said.

"All we can do is sit here in distress, watching [the department] make a huge mess of her life.

"What is happening is criminal and those who have contributed to this problem should be charged."

Tennant Creek staff overloaded with cases

The Tennant Creek child protection office services the entire Barkly region.

Staff there have often had the heaviest case loads.

During one of its busiest periods, March 2012, department statistics showed the office had 112 cases and just one full-time equivalent child protection worker.

The department's own guidelines recommend seven to 14 cases for a remote child protection worker.

In December last year — about the same time as the 11-year-old girl was taken into official care — the Tennant Creek office had a caseload average of 26 per staff member.

The Department of Children and Families refused to talk about the girl's case.