Its important to ensure you aren't too warmly dressed, as underneath that sling is her clothing, your clothing, the sling, natural body heat and there isn't much airflow.

Being such a littley I assume she is bascially cocooned in her sling?

You could try pulling up the top cover (where the pocket is) to have her legs out a bit.
Bringing the side arms in so that not so much of you (and her) is covered.

I too used the HAB from about 3 months of age with my DD and I live in Brisbane, it was summer. I just followed the basic, dressing my lighter and adjusting the coverage of the HAB. I love my HAB.