A man who worked in immigration under the Barack Obama administration says a wall on the Mexican border will stop some people going into the US, but others will find another way.

US Citizenship and Immigration Services former Chief Counsel Stephen Legomski told Mike Hosking all a person has to do to gain access to the US, is to come in as a tourist and stay.

"So given the exorbitant cost of a wall and deploying additional border patrol personnel, it's hard to see how much positive effect that could have in any event."

US President Donald Trump today signed his latest executive order to build the wall along the border with Mexico, which the White House still says Mexico will pay for "one way or another".

"Building this barrier is more than just a campaign promise. It's a common sense first step to really securing our porous border," said White House spokesman Sean Spicer on Wednesday.

"This will stem the flow of drugs, crime, illegal immigration into the United States."

He said the orders will strip federal money from so-called sanctuary cities and end a "catch and release" policy of previous administrations.