The HSE spent up to €22 million on taxis in the first eight months of this year.
It was mainly used to transport patients who were not acutely ill and for moving files between hospitals.
The HSE says the use of taxis in these cases is more cost-effective than using its own vehicles.
But Sinn Féin's health spokesperson, Louise O'Reilly, says taxis are being used too often to transport documents.
"Anyone who works near a hospital will be familiar with the sight of a taxi pulling up and a member of staff coming up and putting a file in it," said Ms O'Reilly.
"We know that some of the budget is being used to transport files between hospitals and between healthcare facilities and the HSE is not monitoring this.
Ms O'Reilly has called on the executive to provide a proper breakdown of the new figures.
"I think they do have to answer those questions and I have asked them on a number of occasions to identify what they believe to be the value for money and yet I have not had any definitive answers from them."
Yesterday, it emerged that the HSE spent nearly €7m on private ambulances in the first eight months of the year.
The HSE's spend on private ambulance has gradually increased in recent years, from €4.3m in 2014 to €8.1m in 2017.
The National Ambulance Service responds to emergency calls 24 hours a day but often struggles to meet the level of demand.
The HSE contracts a number of private companies to assist with deal with less urgent cases.