Two paramedics are being investigated after complaints they used an ambulance to take rubbish to a recycling centre.
Tip users said they were seen unloading garden waste at the centre in St Austell, Cornwall, last week.
The South Western Ambulance Service NHS Trust said the crew was on a compulsory 30-minute unpaid rest break at the time.
It conceded it was neither "appropriate nor acceptable" and said an internal investigation had begun.
In a statement, the trust told BBC News: "Thankfully and most importantly, this situation was in no way detrimental to patient care because the crew were not booked on duty.
"The trust takes matters of this nature very seriously however and would like to make an assurance that the matter will be investigated thoroughly."