How to File Adjustments to Prior GST/QST Returns — Montreal CPA Firm Near You

Every business eventually makes mistakes on a GST or QST return — missing invoices, incorrect tax codes, misreported sales, forgotten credits, duplicate entries, reversed transactions, or software errors. Sometimes the error benefits the government (you overpaid tax), and sometimes it harms you (you owe more). Filing adjustments to prior GST/QST returns is a normal part of compliance, but doing it incorrectly can trigger audits, refund freezes, notices of reassessment, or penalties. This guide explains how to safely and professionally adjust past GST/QST returns using Mackisen’s proven correction process.

Why Adjusting Past Returns Matters
GST/QST are trust taxes. When you file incorrectly, the government assumes your numbers are wrong and may reassess you with penalties and interest. Adjusting past returns promptly ensures your books match your filings, protects your refund eligibility, and prevents future compliance issues.

Legal and Regulatory Framework
GST adjustments fall under the Excise Tax Act, and QST adjustments fall under the Tax Administration Act. CRA’s “Request for Adjustment” system allows corrections for most returns within four years. Revenue Québec allows corrections through the FPZ-500-V adjustment feature in Mon Dossier.
Hickman Motors (1997, SCC) confirms the taxpayer must prove accuracy.
Canderel (1998, SCC) requires filings to match commercial reality.
Lac d’Amiante (2001, SCC) confirms that RQ can reassess prior returns if errors are detected.

What Happens If You Don’t Correct Past Returns
Imagine discovering that for the last six months, your accounting software double-counted GST collected. You filed your returns believing everything was correct. Now your GST account is overstated, and CRA could reassess you for thousands in unpaid refunds. Or imagine realizing that you forgot to claim $20,000 in eligible ITCs/ITRs last year. Without adjustments, you lose those credits permanently. Worse, CRA or RQ may detect discrepancies and initiate an audit, delaying all future refunds. Errors ignored become expensive problems.

Learning Section: When You Must File an Adjustment
You should adjust past GST/QST returns when:
Sales were reported incorrectly
GST/QST collected was wrong
ITCs/ITRs were missed
Suppliers issued late credit notes
Imported or interprovincial purchases were not self-assessed properly
E-commerce platform reports were misinterpreted
Transactions were entered twice
Refunds were denied due to missing documentation
Any correction that affects tax payable or refund requires an adjustment.

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How to Adjust a GST Return (CRA)
Step 1: Log into CRA My Business Account
Step 2: Select your GST/HST (RT) account
Step 3: Choose “Adjust a Return”
Step 4: Select the reporting period
Step 5: Enter corrected figures for sales, GST collected, ITCs, and adjustments
Step 6: Upload supporting documents if requested
Step 7: Submit and save the adjustment confirmation
CRA will then reassess your return and issue a corrected Notice of Assessment.

How to Adjust a QST Return (Revenue Québec)
Step 1: Log into Mon Dossier pour les entreprises
Step 2: Select “Correct a FPZ-500-V Return”
Step 3: Choose the correct period
Step 4: Enter corrected values for sales, QST collected, ITRs, and adjustments
Step 5: Upload documentation if required
Step 6: Submit and save the confirmation
RQ will issue an updated Notice of Assessment reflecting the adjustment.

Documentation Section
Original return
Corrected return
Invoices
Proof of payment
Credit notes
Contracts
Bank statements
Reconciliation worksheets
Internal correction notes
Merchant processor reports
You must keep all documents for six years in case CRA/RQ audits your adjustments.

Common Mistakes to Avoid
Adjusting without reconciliation
Correcting the wrong period
Not keeping documentation to support changes
Claiming ITCs/ITRs without proper invoices
Failing to reverse prior errors completely
Ignoring CRA/RQ follow-up messages
Waiting too long — missing the adjustment window
Not updating your accounting system after corrections

Winning With Revenue Québec and Canada
Mackisen prepares professional adjustment packages, including full reconciliation, documentation indexing, error explanation notes, and proper filing. If CRA or RQ requires additional information, we respond immediately with an organized support file. Our method ensures your adjustments are accepted smoothly and do not trigger unnecessary audits.

Mackisen Service Hub
We provide:
GST and QST adjustment preparation
Reconciliation of prior periods
Documentation organization
CRA/RQ communication
Refund claim recovery
Audit defense
Accounting system correction
Our expert process ensures your adjustments are clean, compliant, and fully supported.

Why Mackisen
With over 35 years of CPA and audit expertise, Mackisen CPA Montreal ensures corrections to GST/QST returns are handled flawlessly. We protect your refunds, correct errors rapidly, and defend your file against any CRA or RQ scrutiny.