Question

How can we rectify our previous financial year mistake in a current financial year if turnover is under or over reported or wrong reporting of tax in gst return 3B ?

Answer

If you have feeded wrong details regarding tax or turnover then you can adjust it in succeeding GSTR-3B on net basis.
In case you have doubt regarding closing of Financial year then proviso to section 39 provides that you can do any rectification in the returns upto the furnishing of annual return or due date of return for the month of September after the end of FY to which invoice relates (which ever earlier).

Circular No. 26/26/2018-GST dated 29.11.2017 clarifies the above as follows:-

4. It is clarified that as return in FORM GSTR-3B do not contain provisions for reporting of differential figures for past month(s), the said figures may be reported on net basis along with the values for current month itself in appropriate tables i.e. Table No. 3.1, 3.2, 4 and 5, as the case may be. It may be noted that while making adjustment in the output tax liability or input tax credit, there can be no negative entries in the FORM GSTR-3B. The amount remaining for adjustment, if any, may be adjusted in the return(s) in FORM GSTR-3B of subsequent month(s) and, in cases where such adjustment is not feasible, refund may be claimed. Where adjustments have been made in FORM GSTR-3B of multiple months, corresponding adjustments in FORM GSTR-1 should also preferably be made in the corresponding months.

Please note that succeeding month means any month comes after the month in which return is filed it does not mean next month only.( Reply dt. 03/06/2018)