Homes and businesses across Victoria have been raided on suspicion of tax "mischief".
The Australian Taxation Office on Thursday confirmed the investigation, targeting "phoenix activity" and tax avoidance.
A day earlier, 11 properties at Melbourne and Shepparton were raided "to gather information as part of a long-term compliance action into a range of tax mischief", deputy commissioner Jeremy Geale said in a statement.
"We are examining a group of tax agents suspected of facilitating phoenix activity and promoting avoidance of tax involving GST, income tax and the failure to remit pay-as-you-go withholding tax payments," he said.
"We suspect the agents have used phoenix techniques to assist clients to avoid paying tax on millions of dollars of income."
On its website, the Australian Securities and Investments Commission defines illegal phoenix activity as the creation of a new company to continue the business of an existing company that is deliberately liquidated to avoid paying taxes, creditors and employee entitlements.
The ATO acted on numerous tip offs, including some from concerned clients of the tax agents.