Former National and ACT leader Don Brash has described Donald Trump as an "appalling" President.

Dr Brash made the comments in an AMA ('ask me anything') session on the ConservativeKiwi section of web forum Reddit on Thursday night.

"Notwithstanding that I agree with some of the things Trump has done, I think he has been an appalling President," he told members of the group.

Dr Brash, like Trump, sits on the right of the political spectrum. But he could only think of "one or two of Trump's policies I agree with", singling out his lowering of the United States' corporate tax rate from 36 to 28 percent.

But the former Governor of the Reserve Bank said Trump can't take credit for the current strong performance of the US economy.

"The US economy has been growing steadily, and US unemployment falling steadily, since the end of the [global financial crisis]," Dr Brash wrote.

"Trump may have prolonged the growth somewhat, but he didn't start it. And to the extent that he has prolonged the growth by running a ginormous fiscal deficit (now running at around a trillion dollars a year) at the height of the boom, that seems to me to have been grossly irresponsible. You run deficits of that size when the economy needs big support, in a recession."

Then Dr Brash stuck the knife in over Trump's constant lies - more than 13,000 since he became President, the last time the Washington Post checked - and his foreign policy.