iMac Messages now only showing +61 iPhone numbers instead of contact names

My iMac Messages (Version 11.0 (5500) is now only displaying the contact number of my iPhone contacts (with +61 prefix) - BUT will display the contact name of android phone owners who message. I have previously tried re-syncing through iCloud, turning on/off etc but nothing is working yet. Interesting to notice the android phone contacts are the ones to be displaying correctly. Syncing with iMac High Sierra 10.13.6, iPhone SE, iPad Pro & MacBook Air. Any ideas much appreciated. Thanks