After completing her studies Tertia worked  on two illustration projects while holding teaching positions. The first one for Songololo Books of her illustration mentor Niki Daly and the other for Quillery Publishers. Marshmallows Monsters and Mice were mentioned as one of the books of the year by “Book Chat” magazine in 1990.

The book is about a little girl who dreams. In the story she learns how to take authority over the monsters in the dream, and tell them to go away.