The level of Poland's longest river, the Vistula, has fallen to a record low, the meteorological office said, leaving sandbanks exposed in Warsaw and water so shallow a moose was filmed walking across it in a section in the countryside.

According to the European Union's climate change monitoring service, the northern hemisphere had its hottest summer since records began, leading to droughts in many areas.

Data from Poland's Institute of Meteorology and Water Management (IMGW) showed that there was 22 cm of water in the river at a measuring point in Warsaw. The previous record low was 26 cm in 2015. The depth of the river in Warsaw is usually 105-250 cm.

"There was no rainfall, there were high temperatures which mean high evaporation, but at the same time warm winters, which meant that there was no thaw, so there was no ... rebuilding of resources, of surface and ground waters" IMGW spokesperson Grzegorz Walijewski said.

In an illustration of how shallow the river has become, reporters from Polsat News filmed a moose walking from one side to the other with the water well below its knees, around 60 km from the capital, near the village of Radwankow.

Poland has a moose population of more than 30,000. It is rare to see them crossing the river's normally fast-flowing deep water.

In 2015, when the Vistula fell to its previous record low level, Polish power grid operator and some electricity producers using river water for open cooling systems had to cut output to avoid overheating of the river that would endanger the fish.

However, the growing share of renewable energy, especially produced by solar farms, and more options to import electricity and hydrotechnical investment made by utilities mean the problem has been alleviated, the Polish power grid said on Monday.

"So for now, there's no indication that even though the water level in the Vistula is very low, it will cause any problem for the power industry," a spokesperson for power grid operator PSE said.