Vitamin D supplementation may benefit atopic dermatitis [allergic skin rash]. We produce vitamin D naturally from direct sun exposure but living in Melbourne or the lower latitudes of Australia, being dark skin or spending more time indoors increases the risk of Vitamin D deficiency and a supplement might be necessary.
Kim MJ, Kim SN, Lee YW, Choe YB, Ahn KJ. Vitamin D Status and Efficacy of Vitamin D Supplementation in Atopic Dermatitis: A Systematic Review and Meta-Analysis. Nutrients. 2016 Dec 3;8(12). pii: E789. https://www.ncbi.nlm.nih.gov/pubmed/27918470