Don't know about Australia (that was someone else), but there are 6 species of Nightjar in Thailand. Frogmouths are similar birds, but not Nightjars. I've seen 1 and heard 3 of Thailand's Nightjars. There are about 6 in North America. I seen and heard 3 of those, including Common Nighthawk.
Had to look this part up, but there are between 60 and 97 species of Nightjar in the world, depending on whether Nighthawks (and others) are included in the family.