"We haven't had a big one like this, and certainly haven't had a shark that's been around for five or six days in a row," he told Macquarie Radio on Thursday.
He said patrol teams on jetskis set out at 7.30am today to look for the shark.
The shark, which has also been sighted by water police at Burwood Beach, seems to be hanging around an area stretching about 12km.
Mr Withers said all reports showed it was a big shark.
"We've had some sightings up really close with the jetski - and it's certainly longer than the jetski and the sled combined - so it is a big shark," he said.
Adult great white sharks grow to about 6.5m long, he said.
Mr Withers said the co-operation from the public has been good since the beaches closed, but admitted some surfers were still willing to risk the waters.
Most summers there were about four or five shark sightings, he said, when the beaches were closed while the shark was herded away by patrols on jetski.
"More times than not we never see the shark again," he said.
Nobbys Beach inspector Paul Bernard told AAP the shark could bite a person in half.
"You don't get a second chance with these things," he said.
Overcast and warm weather combined with increased marine activity could be responsible for the shark's prolonged presence.
- AAP
