This tooth is indeed a fossil, from an extinct Mako shark.

• It comes from an extinct Mako shark called Isurus hastalis, which lived approximately 5  to 30 million years ago during the Miocene & Oligocene periods.
• Isurus was distributed throughout the oceans of the world, with fossil remains found from Belgium to California; from Morocco to Peru. 
• They are also found at a number of sites in Australia. 
• Only the teeth, vertebrae and fin bones of these mega sharks have been found, because shark skeletons are mainly made of cartilage, which decomposes too quickly for it to become fossilised.
• The lack of  serrated edges on this tooth makes it easy to distinguish from the Carcharocles/ Charcharodon species;  Carcharocles megalodon & augustidens  
• The teeth of the augustidens is distinguished from the megalodon by the two small side lobes or blades on the edge of the main blade at its root.