I think it's a dog off-leash beach on the north side of the creek and again to the south of the flagged/patrolled area. You might get pinged by council if you have the dog loose at the main entry to the lake (near the cafes, etc.) There are a couple of good spots to launch up the creek including the park on the wurtulla side just east of the main drag. It's a fun, safe spot to paddle. The following is from council's website, http://www.sunshinecoast.qld.gov.au/sitePage.cfm?code=dog-off-leash-areas :
Currimundi Beach Poatina St, Wurtulla to Currimundi Creek, Currimundi (beach access 249 to 253) Off-leash at all times
Currimundi Beach Gothic Parade to Ann Street Beach access 255 to 262 Off-leash at all times