Rust in the water can occasionally cause rust stains on laundry. These stains can usually be removed.
A few tips:
Wash the laundry again straight away, before it has dried.
Leave the wet, soiled laundry to soak for 1 hour in water containing 1 to a maximum of 2 tablespoons of vitamin C. This is available in powder form from pharmacies and is suitable for wool, cotton and synthetic fibres. If the laundry has already been dried, it is best to soak it overnight.
Wash the laundry again using buttermilk.
Various brands sell stain removers in supermarkets or DIY shops.
Not every stain remover or detergent is effective against rust stains, as the active components may be present only in very small quantities.