Concern has been growing for weeks that the North is building up to a fifth nuclear test ahead of a rare, ruling party congress to be held early next month.
"We assess that they have completed preparations for a fifth nuclear test and can conduct it whenever they decide to," Park said during a meeting with editors of domestic media organisations.
If North Korea does go ahead, it would constitute a dramatic act of defiance in the face of tough UN sanctions imposed after its most recent nuclear test in January.
"North Korea's miscalculation is that by ignoring warnings from the international community and continuing to launch provocations, it will not defend its security but only speed up its own collapse," she added.
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In recent months, the North has claimed a series of breakthroughs in developing what it sees as the ultimate goal of its nuclear weapons programme -- an intercontinental ballistic missile (ICBM), capable of delivering a nuclear warhead to targets across the continental United States.
Earlier this month, leader Kim Jong-Un monitored the test of an engine specifically designed for an ICBM that he said would "guarantee" an eventual strike on the US mainland.
The South's Yonhap news agency today quoted unidentified government sources as saying the North had readied a medium-range Musudan missile for an imminent test launch.
Existing UN resolutions forbid North Korea from the use of any ballistic missile-related technology.
The Musudan is believed to have an estimated range of anywhere between 2,500 and 4,000 kilometres (1,550 to 2,500 miles). The lower range covers the whole of South Korea and Japan, while the upper range would include US military bases on Guam.