President Jacob Zuma flouted the constitution in using public funds to upgrade his private residence and must repay the money, South Africa's top court ruled on Thursday, sparking calls for his impeachment.

The Constitutional Court delivered a damning verdict on Zuma's conduct after a swimming pool, chicken run, cattle enclosure and amphitheatre were built at his rural homestead as so-called "security" measures.

The president had refused an ombudswoman's orders to repay money spent on the upgrades, which became a symbol of alleged corruption and greed within the ruling African National Congress (ANC) party.

Zuma "failed to uphold, defend and respect the constitution as the supreme law of the land," Chief Justice Mogoeng Mogoeng said in a strongly-worded judgement.

The unprecedented rebuke came as Zuma fights back against separate allegations that a wealthy Indian family influenced ministerial appointments in a scandal that has rocked his government.

Zuma has also been battered by the country's sharply declining economy.

But he retains a strong grip on parliament through his dominant leadership of the ANC, and any impeachment bid looked unlikely to succeed.

Public Protector Thuli Madonsela, the country's ombudswoman, ruled in 2014 that Zuma had "benefited unduly" from the work on the Nkandla property in KwaZulu-Natal province, and that he should re-fund some of the money.

The president reacted by ordering two government investigations that cleared his name - including a report by the police minister which concluded that the swimming pool was a fire-fighting precaution.

Mogoeng said on Thursday that Zuma "must personally pay the amount determined by the national treasury."

The work was valued in 2014 at 216mn rand (then $24mn).

"President Jacob Zuma's action amounts to a serious violation of the Constitution, and constitutes grounds for impeachment," the Democratic Alliance (DA), South Africa's main opposition party, said.

It added that it had officially begun the process to impeach Zuma.

Zuma weakened?  

But the ANC controls parliament after winning elections in 2014, and Zuma easily survived a no-confidence vote earlier this month.

A successful vote to impeach Zuma would require a two-thirds majority in the assembly.

The DA brought the case to court along with the far left Economic Freedom Fighters (EFF) party.

"We call on the president to step down with immediate effect," EFF leader Julius Malema said. "We would call upon the ANC to do the right thing and recall the president."

Opposition parties hope Thursday's ruling will bring gains in local elections this year as frustration grows over 25% unemployment and grinding poverty for many black people more than 20 years after the end of apartheid.

Zuma has recently endured renewed corruption allegations after deputy finance minister Mcebisi Jonas said he was offered the top job in the treasury by the Guptas, an Indian business family said to hold huge sway over the president.

The president has faced growing criticism since he sacked two finance ministers within days, triggering a collapse in the rand and a major withdrawal of foreign investors.

"He enjoys less power inside the ANC than he did before firing the finance ministers in December," Aubrey Matshiqi, analyst at the Helen Suzman Foundation, told AFP.

"But the erosion in power he has suffered is not sufficient and does not constitute a direct threat yet to his position."

Zuma, 73, will have completed two terms in 2019 and is not eligible to run for president again.

But the ANC, which led the fight against white-minority rule and has ruled since Nelson Mandela became president in 1994, could replace him ahead of the vote.

Both Zuma and the ANC issued brief statements saying they respected the court's decision and would study its findings.

"The Constitutional Court has restored hope in the constitutional dream for a man on the street to hold the government accountable," Public Protector Thuli Madonsela said after the ruling.