Welcome to the Bunnings Workshop community @murphymazz12. It's fantastic to have you join us and many thanks for your question about which tradesperson to contact.
I'd suggest that if this is an unresolved or reoccurring issue, then it might be wise to get employ the services of some type of specialist. Is this a plaster over brick wall? If it's a solid brick wall, it will probably require moisture detecting equipment to track down the source of the moisture. If it's a plaster over a timber frame, it will be relatively easy for a plumber to cut out a section of plaster to investigate the leak.
Calling a plumber might be the best option, as they'll likely have detection equipment on call and can repair leaking pipes. A damp specialist will be great at isolating the area from where the moisture is coming from, but not necessarily be a plumber able to fix the pipes. Call a plumber first to inspect and he'll let you know if a more specialized service is needed.