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.

Most politicians occasionally bend the truth, but Trump seems to have made lying a way of life.

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."

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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.

"Most politicians occasionally bend the truth, but Trump seems to have made lying a way of life.

"The way he dumped the Kurds in Syria tells the world never to trust the US, and tearing up the Iran deal was arguably worse. I have no brief for the Iranian regime, but the deal which had been worked out by the US, UK, Germany, France, Russia and China, as well as Iran, was being complied with by Iran in every detail. 

When people do take the trouble to read it... they frequently find they agree with every word in it.

"What Trump did by tearing it up was to announce to the world that you should not trust the US. And that's an appalling thing to have done to the US reputation."

The ConservativeKiwi section of Reddit has 333 members. Elsewhere in the AMA, Dr Brash defended his infamously divisive Orewa speech of 2004, saying it lifted the National Party 20 percent in the polls. 

"When people do take the trouble to read it... they frequently find they agree with every word in it."

He also railed against affirmative-action schemes, saying in the US they led to African-Americans being promoted into jobs they weren't qualified for.

"If two people are equally qualified for a job, then perhaps there is a good case for preferring somebody from an 'under-represented group' - perhaps a woman, perhaps a Māori, perhaps an Asian, or whatever. But promoting people simply on the basis of race (or simply on the basis of gender) is a terribly dangerous path."

Dr Brash is presently a spokesperson for group Hobson's Pledge, which wants all race-based rules and laws overturned.