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

1. Email address: your USI will be sent to the email address registered in your USI account.
2. Mobile number: your USI will be sent to the mobile number registered in your USI account.
3. Personal details and security questions: you will need to know and answer your security questions.
4. Personal details and an identification document: you will need to have a valid form of ID, that matches your USI details.

Please refer to the USI government website for more information.