HST/GST Audits for Ontario Small Businesses: What to Expect

For an Ontario small business, an HST/GST audit can feel different from an income tax audit — it touches every invoice you've issued and every input tax credit you've claimed, often across several years at once. The HST/GST audit process is administered federally: even though Ontario harmonized its sales tax into the federal system, it's the CRA — not the Ontario Ministry of Finance — that audits and enforces it under the Excise Tax Act.

This guide walks through how an HST/GST audit typically unfolds and the areas that most often generate problems for small businesses.

How HST/GST Works — A Quick Refresher

Registered businesses charge HST on their taxable supplies and remit it to the CRA, while claiming input tax credits (ITCs) to recover the HST they paid on their own business inputs. In Ontario, HST combines a federal and provincial component; as of mid-2026 the combined rate is 13% — verify the current rate before relying on it, since combined sales tax rates can change. Some supplies — including basic groceries, prescription drugs, and most health, education, and financial services — are exempt or zero-rated, which affects both what you charge and what you can claim.

Getting the mechanics wrong in either direction — undercharging tax you should have collected, or over-claiming ITCs you weren't entitled to — is exactly what an HST/GST audit is designed to catch.

How an HST/GST Audit Typically Unfolds

1. Selection. Businesses can be selected for an HST/GST audit based on industry risk factors, inconsistencies between reported sales and other data sources, a pattern of large ITC claims relative to reported revenue, or as part of a broader income tax audit that expands to cover GST/HST filings.
2. Initial document request. The auditor typically asks for GST/HST returns filed, sales invoices, purchase invoices supporting ITC claims, and bank statements for the periods under review.
3. Reconciliation. The auditor compares reported sales and remittances against your actual books, bank deposits, and third-party information, and tests a sample of ITC claims against supporting invoices.
4. Follow-up and proposal. Discrepancies typically generate follow-up questions, and the CRA will usually issue a proposal setting out preliminary findings before finalizing anything.
5. Reassessment. If the audit results in changes, the CRA issues a Notice of Reassessment for the periods involved, which can include additional tax owing, interest, and in some cases penalties.

The Most Common Problem Areas

Missing or Inadequate ITC Documentation

To claim an input tax credit, you generally need to be able to support it with proper documentation showing the HST/GST was actually paid on a qualifying business expense. A common audit finding is ITCs claimed without adequate supporting invoices, or invoices that don't clearly show the tax registration information needed to substantiate the claim.

Mixing Personal and Business Expenses

Claiming ITCs on expenses that are partly or wholly personal is a frequent problem, particularly for small business owners who run some personal costs through the business. Only the business-use portion of a mixed expense qualifies.

Misclassifying Exempt or Zero-Rated Supplies

Not every sale is taxed the same way. Businesses in sectors bordering exempt or zero-rated categories (certain health services, financial services, or exports) sometimes get the classification wrong, either under-charging tax they should have collected or failing to claim ITCs they were entitled to.

Registration Threshold Issues

Businesses below a certain revenue level aren't required to register for and charge HST/GST, but once that threshold is crossed, registration and collection obligations begin. Because the exact threshold and its rules can be nuanced, confirm your registration obligations directly against current CRA guidance rather than relying on general assumptions — getting this wrong in either direction (charging tax you shouldn't, or not charging tax you should) is a common audit finding.

Related-Party and Intercompany Transactions

Businesses under common ownership sometimes handle HST/GST inconsistently on transactions between related entities. These transactions attract particular audit scrutiny because the tax treatment can be more complex than an arm's-length sale.

How to Prepare Before — and During — an Audit

• [ ] Reconcile your GST/HST returns against your general ledger and bank statements before the auditor does
• [ ] Confirm that every claimed ITC has a supporting invoice showing the required information
• [ ] Separate personal and business expenses cleanly, especially for owner-operator businesses
• [ ] Review any exempt or zero-rated sales to confirm they were classified correctly
• [ ] Document the business purpose of any unusual or related-party transactions at the time they occur
• [ ] Respond to auditor requests within the specific scope asked, and keep a record of everything provided

If You Disagree With the Result

If an HST/GST audit results in a reassessment you believe is wrong, your recourse follows the same general path as an income tax dispute: first a Notice of Objection filed with the CRA's Appeals Branch, and if that doesn't resolve it, an appeal to the Tax Court of Canada. GST/HST appeals to the Tax Court can proceed under the simplified Informal Procedure where the amount in dispute falls within its threshold, which is generally faster and less formal than the General Procedure — though Informal Procedure decisions don't set binding precedent for future cases.

Frequently asked questions