Two women out exercising in Melbourne's north were sexually assaulted by a cyclist who attacked them in broad daylight, police said.

Police believe the two separate assaults in Coburg North may be linked and have issued a warning to women who exercise outdoors in the area to be cautious.

In the latest attack on Boxing Day, a 51-year-old woman was exercising on the Merri Creek bicycle trail about 7.30pm when a man walked past her wheeling a bicycle and exposed himself.

The woman continued walking past the flasher when he sexually assaulted her from behind. She screamed and ran to safety.

Two weeks earlier, on December 15, a woman in her 40s was running along Newlands Road in Coburg North about 1.40pm when she was approached from behind by a man on a dark mountain bike.