The deputy prime minister is facing questions over how his girlfriend and former staff member Vikki Campion left his office last year to take a job with Nationals Senator Matthew Canavan and then with the party's whip Damian Drum.

Senior Nationals senator Nigel Scullion said the party tried to keep skilled staff employed when something happened to an MP.

"We make sure we look after our staff," Senator Scullion told Sky News on Monday.

"My understanding is that Barnaby or Vikki Campion have absolutely nothing to answer for, although they appear to be paying the penalty pretty heavily no matter whether they've done anything or not."

Ms Campion left Mr Canavan's office when he was caught up the citizenship saga to become a senior adviser for Mr Drum, reportedly on $190,000-a-year.

Mr Turnbull said the Nationals were given a specific number of personal staff positions as a share of the government's overall staffing pool.

"The distribution of those staff members between Nationals offices is a matter for the National Party," Mr Turnbull told parliament.

"At no time did the Nationals fill all vacant staffing positions."

Ms Campion is now pregnant with Mr Joyce's child - his fifth - after he split last year from his wife Natalie, the mother of his four children.

Mr Turnbull said Mr Joyce made it clear Ms Campion's employment was not discussed with him or the prime minister's office.

However, the prime minister's office "has an administrative role in informing the Department of Finance of changes", Mr Turnbull said.

A spokesman for the prime minister told AAP Mr Joyce had not breached ministerial standards in regard to the employment of family and partners because Ms Campion was not the deputy prime minister's "partner" at the time of her appointments.

The statement of ministerial standards says family and partners cannot be employed by any members of government "without the prime minister's express approval."

Mr Turnbull confirmed Mr Joyce would be acting prime minister while he was in Washington next week for talks with US President Donald Trump.

When asked if he had confidence in Mr Joyce, Mr Turnbull answered: "Yes".

Asked if Mr Joyce was an appropriate acting prime minister choice, Treasurer Scott Morrison said: "Of course he is".

"While events regarding Barnaby's private life, I'm sure are disappointing ... most importantly to his family and others, that doesn't change the fact that Barnaby, over a long period of time in his public life, has dedicated himself to public service and the people he represents," he told ABC TV.

Senator Scullion said the Nationals "absolutely" backed Mr Joyce.

Nationals senator John Williams said he couldn't judge if Mr Joyce had done anything against the rules.

"Let's see how all of the travel things come out and so on. I just don't know," he told ABC TV.

Mr Joyce appeared to stumble on some infrastructure answers in Monday's Question Time, as he included investment in a Sydney airport and inland rail as part of an answer about Tasmanian investment.

"Isn't the infrastructure minister simply not up to the job that he has been given?" opposition infrastructure spokesman Anthony Albanese said.

Australian Associated Press