A fisheries conservation officer stumbled across one of the frogs in October 2008 while researching an endangered fish species in the Southern Tablelands of New South Wales state.
The officer, Luke Pearce, told The Associated Press he had been walking along a stream trying to catch a southern pygmy perch when he spotted the frog next to the water.
Pearce returned in the same season in 2009 with experts who confirmed it was a colony of around 100 yellow-spotted bell frogs.
Dave Hunter, threatened species officer with the Department of Climate Change and Water, said the find is very important.
"To have found this species that hasn't been seen for 30 years and that professional researchers thought was extinct is great," he said. "It gives us a lot of hope that a lot of other species that we thought were extinct aren't actually extinct -- we just haven't found them."
The find wasn't made public until now to allow enough time to establish conservation measures to protect the frogs from many dangers, including poaching, Hunter said.
The discovery is "as significant in the amphibian world as it would be to discover the Tasmanian tiger, said Sartor, the environment minister.
The last known tiger -- a cousin of the Tasmanian devil -- died in a zoo in 1933, although unconfirmed sightings have been reported since then.
Seven of 216 known Australian frog species have disappeared in the last 30 years.
Mike Tyler, a frog expert at the University of Adelaide, said around a dozen species of Australian frogs are regarded as critically endangered.
"Most of them are on the east coast, mainly in Queensland and New South Wales," he said, but added there are probably other species that never have been identified.
Tyler said the cataloguing of fauna in Australia is still far from complete.
"In the last decade, three new species of frog have been discovered in the Kimberley," he said, referring to a northern region of Western Australia state. "I know of two more in the Northern Territory which haven't even yet been described ... one of the specimens is sitting here on my desk looking at me."