I do recommend Imogene. Kathryn's shop lives inside an antiques store at the end of the strip of Clifton Hill shops. Her contact details are on the internet. Clifton Hill is a suburb close to th city.
Each of my three ropes I bought from Kathryn (the two mixed south seas and the drop Tahitians) I bought from off her neck! She is a properly knowledgable pearl person. She also has lovely sapphire rings and other lovely things. My advice would be to call ahead and ask for what you might be interested in seeing in case she needs to bring it from offsite or speak to her suppliers. I wouldn't usually recommend a bricks and mortar shop but this is no usual jewellery store with clueless salespeople or poor quality pearls, and it doesn't have huge overheads to maintain.
I do like Jan Logan in Collins St, but this often comes with Collins St prices. The store is at the top end of town with Chanel, Prada etc. I bought the huge roundish Tahitians rope from there but it was a one-off.