Sounds like it could be an infection and possibly have an underlying allergy.

Speak with your vet about using a using a medicated shampoo such as Malaseb® and also whether Barazone® Medicated Conditioner might be right for your dog. 

One of the causes of hair loss and itchy skin is a fungal infection called ringworm. It’s important that you speak with your vet about your pet as this can also spread to people as well. Malaseb® can help both treat your pet and reduce contamination in the home if it does have ringworm.

Dermcare Barazone® Leave-on Conditioner is only available under veterinary authorisation. Registered pursuant to the ACVM Act 1997, No. A011938. Dermcare Malaseb® Medicated Shampoo is registered pursuant to the ACVM Act 1997, No. A7641. See www.foodsafety.govt.nz for registration conditions.