There has been no official notfication on turnover limit of audit under GST. GST audit by a chartered accountant or cost accountant will be compulsory if turnover exceeds prescribed limit.
However revised return rules issued by government do mention 2 crore as audit limit.
Rule 21(2) of revised return rules prescribed that:
Every registered person whose aggregate turnover during a financial year exceeds one crore rupees shall get his accounts audited as specified under sub-section (5) of section 35 and he shall furnish a copy of audited annual accounts and a reconciliation statement, duly certified, in FORM GSTR-9B, electronically through the Common Portal either directly or through a Facilitation Centre notified by the Commissioner.
So as of now we can consider the rules to be final and assume that turnover for audit will be more than 2 crore.