The fact it is the same tune is unsurprising. Per 'Wiki' on "The carnival is over":
The main tune is taken from a Russian folk song about Stenka Razin known as "Iz-za ostrova na strezhen" or "Volga, Volga mat' rodnaya". The song became popular in Russia as early as 1890s.[4] It was performed by the Osipov State Russian Folk Orchestra (balalaikas and domras) during their 1967 tour of Australia. The tune is also used in a Dutch hymn "Vol Verwachting Blijf Ik Uitzien", and a Dutch nursery rhyme "Aan de Oever van de Rotte".

Tom Springfield adapted the melody from the Russian folk song, and also wrote the remaining music used in the song, as well as writing the lyrics, after a trip to Brazil, where he witnessed the Carnival in Rio.