Humpback and southern right whales migrate along Australia's east coast each year, passing Sydney as they head north away from Antarctica for the coldest months of June and July. They return south between September to November.
"We're expecting a really great season this year," New South Wales National Parks and Wildlife Service whale expert Geoff Ross said.
"The large number of whales that have been passing over the last couple of days indicate the peak days towards the first week of July will be very big days."
Ross said the increase in whale numbers, predicted to be a 10 percent rise this year, was partially the result of a whale baby boom as the animal continues its recovery from past decades of whaling.
"(They) are at the maximum of their reproductive capacity so they're really booming at the moment, producing lots of young," he said.
Australia is a strong opponent of whaling in the Southern Oceans and New South Wales Environment Minister Robyn Parker said conservation efforts had also helped grow whale numbers.
"This is a natural increase because over the years... we've improved the marine environment," she said.
"We're very confident we're doing great things to make the water quality better and certainly make it more conducive to whales."
Whales, including minke and fin whales, come close to the New South Wales coast during their northern migration.
Volunteers count the numbers of whales spotted from Sydney's Botany Bay during the season, and the figures have been growing between eight and 10 percent each year over the past decade.