The technology giant said it uses language experts around the world to study a small number of audio "snippets" from users.
Google said this work helps with developing voice recognition and other technology in its Google Assistant artificial intelligence system, which is used in its Google Home smart speakers and Android smartphones.
The assistant understands and responds to voice commands given to it, answering queries about the news and weather as well as being able to control other internet-connected devices around the home.
In a statement, the company said a small number of anonymous recordings were transcribed by its experts, and revealed that an investigation had been launched after some Dutch audio data had been leaked.
"We partner with language experts around the world to improve speech technology by transcribing a small set of queries - this work is critical to developing technology that powers products like the Google Assistant," Google said.
"Language experts only review around 0.2 per cent of all audio snippets, and these snippets are not associated with user accounts as part of the review process.
"We just learned that one of these reviewers has violated our data security policies by leaking confidential Dutch audio data.
"Our Security and Privacy Response teams have been activated on this issue, are investigating, and we will take action.
"We are conducting a full review of our safeguards in this space to prevent misconduct like this from happening again."
Earlier this year, a report from Bloomberg revealed fellow tech giant Amazon also listens to some recordings of customer interactions with its voice-based assistant Alexa.