President Jacob Zuma's fifth wife, Thobeka Madiba-Zuma, fell into a trap of "post now and regret later", according to a social media law expert.

This follows a social media storm that she unleashed on Friday when she defended her husband in an Instagram post when she posted a picture of them in a private jet and wrote a caption in isiZulu.

"Don't fight with someone who isn't fighting back, things are about to get ugly," she wrote.

After a flurry of criticism against her post, Madiba-Zuma shot back at her critics saying her husband did not join the ANC in 1991.

"He left his home at a tender age to fight apartheid. He was prepared to pay the ultimate price. He will finish what he started because he does not take orders beyond the Atlantic Ocean," part of her response read.

But her post became more popular and started trending on the internet when Zuma's son Edward released a statement condemning Madiba-Zuma's post as "unfortunate and regrettable".

Social media law specialist Emma Sadlier said that like many other people, the first lady fell into a trap of posting first and thinking later.

"It is stupid how people, ordinary and prominent, often fall into that trap.

"In most cases, people find themselves on the wrong side of the law just because of what they write on social media," Sadlier said.

She said the president's wife was lucky because what she wrote does not have legal implications.

"Officials have to be careful of their social media usage, especially when it comes to official matters and politics," she said and cited US president Donald Trump as an example of a person who recklessly uses social media platforms without repercussions.

"He tweets a lot and sometimes on things that are not related to the White House but are his personal opinions.

"He angers people, but what he writes is mostly not legally binding. Therefore, he gets away with the stupid things he tweets," she said.