What makes a return defective, and the 15-day clock
Where the assessing officer considers a return defective, Section 139(9) requires the department to intimate the defect and give you 15 days to rectify it, extendable if you apply. The consequence of not curing it in time is the sharp part: the return is treated as an invalid return, meaning it is as if you never filed, so the filing and any refund fall away and you are exposed to the non-filer consequences that follow from that.
There is a small mercy: if you cure the defect after the 15 days but before the assessment is actually made, the officer may condone the delay and treat the return as valid. But that is discretionary, so the safe course is always to fix it inside the window rather than rely on condonation.
The defects that catch NRIs
For non-residents the defects are usually structural rather than about the numbers. The most common is the wrong ITR form: a non-resident cannot use ITR-1, so a return filed on it is defective and has to be redone on ITR-2 or ITR-3 depending on your income. Another frequent one is a missing schedule, the residential-status details, the capital-gains schedule, or a foreign-asset schedule where it applies, left blank or omitted.
The third common trigger is a mismatch between the tax and TDS claimed and what the department has on record, where the return claims a credit that does not reconcile with your Form 26AS. Each of these is a fixable defect, but each requires the return to be corrected and refiled correctly, not just an explanation, which is why the right ITR form and complete schedules matter from the start.
The related power to call for information
It is worth separating the defective-return notice from another notice NRIs sometimes receive, a request under Section 133(6). That is a power to call for information or documents, such as bank statements or a sale deed or the source of funds, and it can be used even where no assessment is pending, subject to senior approval in that case.
Unlike a 139(9) notice, a 133(6) request is not about a defect in your return; it is the department gathering information about a transaction. The right response is to treat it as genuine and provide the information sought within the time given, because non-compliance carries its own penalty. A practising CA handles both, curing a 139(9) defect within the window and answering a 133(6) request with the right documents, from the Indian side under your authorisation.