Those months have the least wind strength and from the wrong direction for Brisbane as you are in our winter months. You may be lucky and score a week of strong wind as a low pressure forms and moves down the coast or if we have a ealy storm season you may get some NE winds in August. Other than that you will just need to be lucky and wait for the occasional SE. The westerly winds we have a lot of at this time of year blow offshore and are gusty and not prdictable and are a pain.
So I would bring a 11 and stay on the sunny or Gold Coast and take up surfing.

Brisbane and further south to Gold Coast
http://www.seqka.com.au/page/kiting-locations

North of here (Sunshine Coast) - look at each river mouth on google earth - Caloundra , Maroochydore, Noosa figure out the wind direction they take. When the tide is flowing out you can get a 4 knot boost to the wind strength (those months you can need it). That is why SEABREEZE graphs showing the tide are great.
The consistent winds are way way north in the trades. The AKSA open is in Townsville (North Qld) in June to catch good winds.
www.aksa.com.au/Default.aspx?pageId=651617&eventId=423144&EventViewMode=EventDetails

Check this for historic records.....
reg.bom.gov.au/climate/averages/wind/selection_map.shtml
NB there can be big difference between 9AM and 3PM caused by seabreezes (mostly in summer).
Hope U have a nice stay
AP