The eight-month-old boy was suffering from eczema when his mother consulted the naturopath in April for advice about alternative health treatments for her son, police said.

Police will allege Marilyn Bodnar, 59, who is also a registered nurse and a midwife, advised the child's mother to stop all medical and dermatological treatments for the child.

The following month, on May 28, the baby boy was admitted to Westmead Hospital suffering from severe malnourishment and developmental issues.

He had lost more than one kilogram in weight and was "near death", a NSW Police spokesman said.

The baby was only released from The Children's Hospital at Westmead on Wednesday.

Child abuse squad detectives went to a property in Leppington, in Sydney's south-west, and arrested the self proclaimed naturopath about 7.30am on Thursday.

Ms Bodnar was taken to Narellan police station and charged with recklessly causing grievous bodily harm in principle in the second degree, and aiding, abetting or failing to provide for a child, causing danger of death.

The baby's mother is already before the courts on charges of recklessly causing grievous bodily harm, and failing to provide for a child, causing danger of death.

Naturopathy is an unregulated profession in Australia and there are no legalities in claiming you are a naturopath when you do not have any qualifications.

Ms Bodnar is not accredited as a naturopath with the Australian Traditional Medicine Society, Australian Natural Therapists Association or National Herbalists Association of Australia.

Detectives say they are continuing their investigation into the naturopath's Leppington practice, particularly in relation to the treatment of children.

The baby's mother and Ms Bodnar were both expected to appear before Fairfield Local Court later this month.

They are asking anyone who can assist them with information to contact Crime Stoppers on 18000 333 000 or to use the Crime Stoppers online reporting page.