In Melbourne, the planet's path through the gap between the Earth and the sun could be watched using a filtered telescope from 6.12am (AEDT).
The event, which occurs about 13 times a century but is only visible about six times from any one point on the earth, will continue until 11.10am local time.
The weather plays a major role in the spectacle, which is why Astronomical Society of Victoria vice president Perry Vlahos was so happy with today's sunny and clear conditions in Melbourne.
"If the weather is not playing the game, you miss it," Mr Vlahos said.
Several hundred Victorians are expected to file through the Melbourne Observatory to catch a glimpse of the event today.
Mr Vlahos, who watched from his own telescope at his house, described the scene by comparing it to a clock face.
"Mercury is like a tiny little black dot up against the image of the sun's surface ... it would be going from about two o'clock through to about ten o'clock," he said.
"Mercury is between us and the sun in an absolute straight line ... It is an eclipse of sorts except that it doesn't cover the whole of the sun."
Mr Vlahos warned people not to try to look at the sun without the safety of a filtered telescope, as it could cause blindness and he reassured those who missed the show that it will come around again - eventually.
Astronomers calculate that it will happen again in November, 2032.
"I've seen one of these before," Mr Vlahos said. "It acts as a way of defining your life because you know what you were doing the last time you saw one."
Mercury, which at its farthest orbit is 77 million kilometres from earth, was first observed crossing the face of the sun in 1631.