South Africa's democratic and constitutional health relies on a more competitive political sphere, former Democratic Alliance (DA) leader Tony Leon said.

"When that happens, as it surely will pretty soon, then our fine constitutional prospectus will start to live and breathe again and be rescued from the torpor of one-party domination," he said in a speech prepared for delivery in Cape Town on Thursday.

The former South African ambassador to Argentina said when he led the DA between 1999 and 2007, the opposition was going up against staunch ANC history, as a party that held all the moral and political high ground.

At that time, he said the African National Congress presented a formidable, often frightening, unity in the face of any opposition, whether external or internal.

"Today, the opposite is true: The opposition has the wind behind its back and the ANC juggernaut is showing signs of decay and sclerosis – the symptoms of a house divided whose inevitable right to rule is now under both question and strain."

Leon said a decade ago he had warned against certain "ANC-sponsored" concepts and practices such as the national democratic revolution, cadre deployment and black economic empowerment.

His pronouncements led to him being called anti-transformational, the voice of white privilege and the fight-back king.

"Now, an entire chorus, including some significant black intellectuals, media editors and trade unionists, are singing from the same hymn sheet, often in far more strident and less polite notes than any I had sounded from my perch as leader of the opposition."

Leon reflected on his three-year stint in the embassy, saying he would scour the Internet daily for news from his homeland.

He said that despite reading numerous doom-and-gloom stories, there were signs of advancement through the harshest of "political winters".

An example was the continued tenacity of the judiciary to find against the government in significant judgments, despite being assaulted and questioned from various angles.

"I do not believe, incidentally, that South Africa is about to fall off the cliff and plunge into a failed state scenario," Leon said.

There were too many institutions and feedback mechanisms for the country to follow the same path as Zimbabwe.

However, it was not enough for the country to muddle along in the hope it would get back on the path set out in 1994.

"Our task as citizens is to engage with the complexity and interdict our weaknesses. As simple, and as complicated, as that."