Are TransferWise borderless account balances visible to tax authorities?

They are not visible to the government in the traditional sense. Both United Kingdom (from where TransferWise is based) and Australia, are a signatory to the OECD Common Reporting Standards (CRS), which allows governments to report to another government the list of its citizens who may have an account in a foreign country. The goal of CRS is to prevent tax evasion.

To answer your question: The government of Australia can ask TransferWise a list of all its Australian citizens who maintain an account with TW, and TW would legally have to comply with the request.

OECD CRS Wiki: Common Reporting Standard | Wikiwand

Clarification by TransferWise on the same: Borderless accounts and tax | TransferWise Help Centre