Every mature Scribbly Gum (Eucalyptus racemosa) I’ve ever seen has these scribbles on the bark, and the scribbles are characteristic of this and one other eucalypt species (E. haemostoma).

“These zigzag tracks are tunnels made by the larvae of the Scribbly Gum Moth (Ogmograptis scribula) and follow the insect's life cycle. Eggs are laid between layers of old and new bark. The larvae burrow into the new bark and, as the old bark falls away, the trails are revealed. The diameters of the tunnels increase as the larvae grow, and the ends of the tracks are where the larvae stopped to pupate.”
http://en.wikipedia.org/wiki/Eucalyptus_haemastoma)

And:

“When the cork cambium starts to produce cork to shed the outer bark it produces scar tissue in response to the feeding of the caterpillar, filling the doubled up part of the larval tunnel with highly nutritious, thin-walled cells.
These replacement cells are ideal food for the caterpillar which moults into its final life stage with legs, turns around and eats its way back along the way it has come. It now grows rapidly to maturity and leaves the tree to spin a cocoon at its base, where it pupates.”
http://www.csiro.au/Outcomes/Environment/Biodiversity/Scribbly-gums-interpreting-their-scribbles/Scribbly-gum-moth-research.aspx)

(web-sites downloaded 2 March 2014)