A eucalyptus leaf is a vegan food item. Eating an uncursed one will cure sickness and nausea. It can also be applied as a whistle, and acts as a magic whistle if blessed and a tin whistle otherwise. There is a 2% chance that a blessed eucalyptus leaf will become unblessed after being used as a whistle.
Eucalyptus leaves are fairly rare, making up only 0.3% of all food items. You have roughly a 29.4% chance of finding any at Medusa's island or above, in the Gnomish Mines, or Sokoban, excluding the quest and all death drops. There is a 4.7% chance of some in Sokoban. Minetown has a 7.2% chance of eucalyptus leaves, mostly due to 35.1% chance by kicking and then cutting down both trees in Bustling Town. Rangers are nearly guaranteed to obtain some from their quest's trees. Pacifists should know that eucalyptus leaves are not generated in any monster's inventory, as opposed to death drops.
The effect of using a eucalyptus leaf as a magic whistle seems to be based on the Australian 1960s TV show Skippy the Bush Kangaroo, which had a recognisable theme call played on a eucalyptus leaf. Skippy came bounding through the bush to whomever played the eucalyptus leaf.