The lockdown took effect at midnight on Thursday March 26 and would have been lifted next week Thursday 16 April at midnight.
His address to the nation followed a meeting of the national coronavirus command council held on Wednesday and further consultations with various social partners, the presidency said earlier on Thursday.
By Thursday, more than 1.5-million people globally had been infected with the novel coronavirus, according to the Johns Hopkins tracker, while more than 90,000 people had died.
Ramaphosa said while it was too early to make a definitive analysis, there was sufficient evidence that SA’s lockdown was working.
He said that in the two weeks before the lockdown, the average daily increase in cases was about 42%, but since then had dropped to about 4%.
He said while the government recognised the need to expand testing, the numbers did indicate “real progress”.
Ramaphosa announced that he, deputy president David Mabuza and all cabinet ministers would each take a one-third cut in their salaries for the next three months, and would donate that to the Solidarity Fund. Provincial premiers would do the same.
“We are calling on other public office-bearers and executives of large companies to make a similar gesture and to further increase the reach of this national effort,” he said.