Malcolm Turnbull stood outside the front gates of the White House and despite the prime minister being half a world away from Canberra he could not avoid questions about his embattled deputy prime minister Barnaby Joyce.

On Thursday (early Friday AEDT) Mr Turnbull repeatedly declined to say if he still supported Mr Joyce as his deputy prime minister.

"These are matters for the leadership of the National Party," Mr Turnbull eventually answered.

Mr Turnbull landed in Washington DC on Wednesday to lead what is the largest and most significant Australian delegation, including state premiers and business leaders, to visit the US.

He met with Treasury Secretary Stephen Mnuchin and Homeland Security Secretary Kirstjen Nielsen Thursday morning and after speaking to reporters was scheduled to meet with new Federal Reserve Chairman Jerome Powell and Joint Chiefs Chairman General Joseph Dunford.

Mr Turnbull and wife Lucy will spend most of Friday at the White House with US President Donald Trump and First Lady Melania.

Fierce debate over gun control has taken over the US after last week's Florida school massacre and Mr Trump has signalled he will look at introducing stricter measures.

Mr Turnbull will likely face questions from the White House press corps and possibly Mr Trump on Friday about Australia's success in responding to the Port Arthur massacre more than 20 years ago.

Mr Turnbull said he would be reluctant to wade into the US gun debate.

"We'll leave the domestic political debate to the Americans," Mr Turnbull said.

Mr Turnbull was buoyed by his discussion with Mr Mnuchin about Mr Trump's recently introduced tax cuts.

"The reality is the experience here in the US is they are seeing strong economic growth because of those tax cuts and underlines the importance of completing our tax reform program," Mr Turnbull said.