The Grass Roots Music Festival is held at the bandstand (next to the Japanese Gardens) at the Brisbane Botanical Gardens in Mount Coot-tha, Toowong. 

The address is: Brisbane Botanical Gardens Mount Coot-tha, Mount Coot-tha Road, Toowong, QLD, 4068

Put the address into google maps for directions -- http://maps.google.com.au/

Here is another link to the venue:

http://www.brisbane.qld.gov.au/facilities-recreation/parks-gardens/brisbane-botanic-gardens-mt-coot-tha/index.htm