Who is the e-mail from?
Did the e-mail not provide further information on how/who to contact regarding the issue?
Its more likely a hacker than Optus so don't select any links in the email. Always log into financial institutions completely on your own.
But instead of the public forum contact Optus direct (LIVECHAT or PHONE) and ask if there's an issue.
If you want to discus the email more here then as Ray says, a few more details would be nice.