Former prime minister John Howard has slammed US President Donald Trump as an "unpredictable" leader and lamented his communication style on Twitter.

Mr Howard, the country's second-longest serving prime minister, took aim on Tuesday at Mr Trump's use of social media to engage with constituents.

"I worry about some of his communication methods," Mr Howard told the International Bar Association Conference in Sydney.

"It's not a communication method I would have adopted. I wish he didn't tweet but he's not going to stop."