The May volumes were also down 1.7 percent from April, the data showed. The last time monthly sales were lower this year was in February.

Vessel arrivals coming to Singapore for refuelling, also known as bunkering, also fell 3.1 percent from a year ago to 3,513 in May.

Before May, the volumes of bunker fuel each ship took have mainly been higher than the same period last year, demonstrating shipowners' confidence in refuelling in Singapore since the introduction of mass-flow-meters (MFM) at the start of the year which are designed to minimise inaccuracies and errors in measuring the delivered quantity of bunkers.

Since Jan. 1, when Singapore became the world's first port to mandate the use of MFMs, bunker fuel sales volumes have consistently set record numbers.

In January, bunker fuel sales reached an all-time high of 4.462 million tonnes, while February, March and April have each posted record volumes for their respective months.