They are saying Nepean Hwy, from mordi to frankston will be closed from 3am to 6:30 pm, I would expect beach rd to be closed to at least then or later.
From the Frankston triathlon site: Athletes will follow the beach pathway to the Frankston SLSC where they will turn East and follow the creek back towards the Transition Area. Just prior to the transition area the course crosses the creek and follows the paths until they reach the Nepean Highway. Athletes will then run on the left lane of the highway to Mordialloc. Once at Mordialloc athletes will follow the pathway to the St. Kilda Sea Baths.
Fortunately, the wind forecast for Sunday is not good (at this stage).