GST Audit is applicable when your Turnover in FY 2017-18 exceeds Rs.2 Crore as per the Provision of Section 35(5) of the CGST Act 2017 read with Rule 80(3) of the CGST Rules 2017.
The last date to file GSTR 9 ( ANNUAL RETURN ) is 31.12.2018..
I also agree with Raja Sir's view regarding Extension of Due Date of GSTR 9. Since it is the first Annual Return under GST, there may be some difficulties for Trade and Industries to file ANNUAL RETURN and also GSTR 9 is not yet available in the GST common portal. Only the GSTR 9 tab has been enabled.