A 15-month-old child held in immigration detention in Australia since birth despite the protests of the United Nations has been hospitalised, advocates say.

Isabella Lee Pin Loong was transported to the Northern hospital, Epping, on Friday suffering what her mother’s lawyer, Alison Battisson, described as a “chronic fever”.

On Tuesday the hospital confirmed that Isabella, who has been held at the Melbourne Immigration Transit Accommodation centre since her birth, was being “treated for a medical condition as an inpatient”.

Ambulance Victoria said it would not comment on people transported to hospital from the immigration centre – despite doing so in previous cases.

Battisson told Guardian Australia that there have been concerns about the child’s welfare for more than a month, and that on Friday evening she was hospitalised with a fever.

“Isabella has spent her entire 15-month life in detention and throughout that time we have raised a number of concerns about the appropriateness of detention for raising a child,” Battisson, the director of the advocate group Human Rights For All, said.

In a statement on Wednesday, Australian Border Force confirmed an infant was being treated at the Northern hospital for influenza but denied the child was “detained” in the immigration facility.

“Her mother is detained and her father lives in the community,” the statement said.

“The infant has been living with her mother at her request, but may come and go from the facility and stay in the community with her family.”

“A range of care, welfare and support arrangements are in place to provide for the needs of children and young people in detention.”

The United Nations has previously called for both Isabella and her Vietnamese mother to be released by the Australian government, saying their detention contravened a number of articles of the Universal Declaration of Human Rights and the International Covenant on Civil and Political Rights.

In June the UN working group on arbitrary detention, which noted Isabella had previously been hospitalised, said her detention and that of her mother, Huyen Thu Thi Tran, was arbitrary and they should be released and receive compensation.

“Taking into account all the circumstances of the case, the appropriate remedy would be to release Huyen Thu Thi Tran and Isabella Lee Pin Loong immediately and to accord them an enforceable right to compensation and other reparations, in accordance with international law,” it said.

The working group rejected the government’s assertion that Isabella was not detained, saying Tran had little choice if she wanted to “see her newborn baby and look after her”.

Battisson said Isabella had been suffering from ongoing fevers for about six weeks, and that she had previously called ambulances to the detention centre to ensure she was receiving proper care.

“Our concerns have been about Isabella’s wellbeing, the appropriateness of medications in the centre and her mother’s inability to maintain appropriate sanitation and well-being there,” she told Guardian Australia.

Tran arrived at Christmas Island by boat in 2011 after fleeing religious persecution.

She was initially detained for 17 months and placed in community detention, from which she absconded. After being taken into detention in November 2017, the UN report noted, her “mental and physical health have deteriorated” and Isabella experienced a number of health issues.

It found Isabella “exhibits signs of attachment related anxiety and is at risk of developmental problems due to her mother’s depression and her prolonged detention”.

“The lack of positive emotional interactions in detention is also likely to have an ongoing negative impact on the infant’s development,” the report noted.

It also wrote that Isabella “has reportedly also experienced illness in detention and been hospitalised”.

It comes after a spate of incidents at the Melbourne immigration centre.

At about midnight on Friday – the same day Isabella was transported to hospital – a 23-year-old Afghan man died at the centre. This week police said his death was not being treated as suspicious but authorities have not provided any other detail about the cause of death.

In the early hours of Monday morning, another 23-year-old Afghan man was hospitalised after he attempted to set himself on fire inside the centre.

The ABF said the infant was first hospitalised on 11 July but released without treatment, and was visited in the detention facility by a child wellbeing officer the next day.

“On 14 July medical professionals at the Melbourne Immigration Transit Accommodation (MITA)—adjacent to the Broadmeadows residential precinct—again assessed her condition and referred her to the Northern hospital,” the statement said.

“In the best interests of the infant, the department ensured that her mother was able to accompany her to hospital and she remains with her.”