Home Affairs head Mike Pezzullo was one of the first to front Senate estimates on Monday.

It's been up and running for only weeks, but his new department is part of one of the largest government portfolios.

Having brought several security agencies into its fold, and if legislation passes letting ASIO join, the Home Affairs portfolio will be home to 23,000 public servants.

Mr Pezzullo was also quizzed on the investigation into Roman Quaedvlieg, the head of the Australian Border Force who has been on leave since May last year, following claims he helped his girlfriend - an ABF staff member - get a job at Sydney Airport.

It was revealed the Prime Minister's department has had a corruption watchdog's report into abuse of power allegations for at least five months while Mr Quaedvlieg has been on full pay earning hundreds of thousands of dollars.

Like estimates in October, senators grilled the Australian Public Service Commissioner about his email correspondence to his former colleagues at the Institute of Public Affairs.

John Lloyd defended an email to his former IPA colleagues about Labor Senator Penny Wong, saying it was a "natural" reaction to do so.

In a heated Senate estimates hearing on Monday, the public service boss said that as the comments were not about policy or made publicly, they were appropriate.

"When someone goes publicly and casts critical comment about your appointment it's normal to share it with family and friends, you know, you don't go 'Gosh the senator has said it and I can't talk about it with anybody'," Mr Lloyd said.

Before John Lloyd was grilled on his emails, he made an opening statement saying how proud he was of the public service and hitting back at media coverage of the work public servants do:

Barnaby Joyce was the champion within the Coalition for the Australian Pesticides and Veterinary Medicines Authority's move to his electorate, New England. Now that he's moved to the backbench, Labor called for the government to renege on the plan. Agriculture minister David Littleproud on Tuesday said no - the move was full steam ahead.

At estimates, the APVMA revealed that day it will spend nearly $1 million on leasing and fitting out its temporary digs in Armidale for more staff as it prepares to move into its permanent new home by mid-2019.

Chief executive Chris Parker told a Senate estimates hearing that $128,000 of the sum would go to consultancy costs, and fees for authorising the building works, while another $231,000 would cover construction.

The spending, which will let the authority house 55 staff at its temporary home, would also cover equipment to be used in its permanent new building in Armidale and not just its interim building shared with the Department of Human Services, Dr Parker said.

Digital Transformation Agency boss Gavin Slater fronted estimates on Tuesday with something to report on the government's online marketplace for IT contracts.

Small businesses had told the DTA they hadn't been able to find work on Digital Marketplace, he said.

The Coalition government wants more smaller businesses getting IT work, and this feedback won't be encouraging.

Mr Slater said he wanted to increase their participation in government work and, in light of results, was not satisfied with the level of performance for Digital Marketplace.

Stability at the Department of the Prime Minister and Cabinet has been questioned after it was revealed that 28 of the senior executive service were "acting" in their roles.

In an estimates hearing on Monday night, Labor senator Penny Wong said the rate at which positions were being filled on an acting basis was "extraordinary".