Check the Forgotten my USI webpage, where you will be able to retrieve your USI using one of the following: 

• Email address: your USI will be sent to the email address registered in your USI account
• Mobile number: your USI will be sent to the mobile number registered in your USI account
• Personal details and questions: you will need to know and answer your check questions
• Personal details and an identification document: you will need to have a valid form of ID