From troop deployments to the maximum running speed of a giraffe, Senate estimates hearings this week have traversed a vast sweep of material.

Hundreds of hours of hearings over two weeks of budget estimates have now wound up — although plenty of the inquiries were "taken on notice" and will not be answered for weeks.

Political correspondent Louise Yaxley examines what we learned during the final week of a fortnight of estimates hearings.

Additional troops deployed in Afghanistan

Defence Minister Marise Payne did not wait for questioning to reveal that the Government will send 30 more military personnel to Afghanistan, taking the total Australian contribution to 300.

Their roles will be to "train, advise and assist" rather than being sent into combat. But the massive bombing in central Kabul on Wednesday revealed how perilous any time spent in Afghanistan could be.

Meanwhile, hours of questioning was required to glean the latest on the Navy's two largest warships. They have been stuck in Sydney since March because of problems with their propulsion systems.

The Navy said they should both be working again by October — but could not say how much the repairs would cost.

Defence Minister Marise Payne confirmed the Government would send extra troops in response to a NATO request. ( ABC News: Marco Catalano )

ATO responds to tax fraud scandal

Tax commissioner Chris Jordan also poured out his news without waiting for the grilling.

He told them an alleged $130 million conspiracy to defraud the Tax Office had struck at the heart of the organisation.

His appearance before the committee was the first time he had spoken about the Plutus scandal, which has caught up one of the ATO's senior officials, Michael Cranston. His son Adam has been charged with conspiracy to defraud the Commonwealth.

Michael Cranston has been suspended without pay and summonsed to appear in court for abuse of public office.

Sorry, this video has expired The tax commissioner gives an insight into how hard it can be for the ATO to catch fraudsters

Mr Jordan gave the senators an insight into how hard it could be for the ATO to catch fraudsters, when they adopted tactics like nominating vagrants as company directors.

He said keeping track of a suspect's friends and family to ensure they were not being used as dummy directors was relatively easy, but linking them to strangers was much harder.

"But if they go out on the street and basically hang outside Centrelink and say, 'Do you want $1,000 bucks a week for something' — well, it is really hard," Mr Jordan said.

Chris Jordan said it was difficult when fraudsters nominated vagrants as company directors. ( ABC News: Marco Catalano )

Ring ring ring — why don't they answer the 42 million phone calls?

Welfare recipients were at the centre of much of this week's questioning. It emerged that many have been annoyed when calls to Centrelink are not answered.

Officials revealed that 42 million phone calls had gone unanswered since July last year.

Frustrated officials suggested many people were using a redialling app on their phone and that was inflating the number.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Listen Duration: 2 minutes 42 seconds 2 m 42 s Forty-two million calls to Centrelink get a busy signal Download 5 MB

But Greens senator Rachel Siewert pointed out that not getting an answer when they called would be exasperating for those trying to get through.

Estimates also exposed the gaps in the details about a plan to drug test some welfare recipients.

Whether it is their hair, saliva or urine that is checked, is not yet clear — nor was there an answer on which parts of the nation would be chosen for the trials.

A blunt discussion on climate change

The Chief Scientist showed himself prepared to give a direct answer to One Nation senator Malcolm Roberts.

In his first speech to Parliament, Senator Roberts said: "Like Socrates, I love asking questions to get to the truth".

He pressed Chief Scientist Alan Finkel about the need to be open-minded.

"Not so open minded that your brain leaks out," Dr Finkel responded.

Sorry, this video has expired Chief Scientist Alan Finkel responds to climate sceptic Senator Malcolm Roberts

Senator Roberts, who disputes accepted scientific views about climate science, also tried to make a point about potential flaws in peer reviewed scientific findings by citing a hoax about "the conceptual penis being the driver behind much of climate change".

That alarmed Science Minister Arthur Sinodinos, who was annoyed that Senator Roberts was suggesting that all peer reviewed papers were in some way fraudulent.

Giraffe rescue

The drug education giraffe Healthy Harold appeared to be no more when Health Department officials revealed it would not get further funding from the Health Department.

Angst was palpable among those of the appropriate generation — now adults — as they struggled with the demise.

But just days later, Education Minister Simon Birmingham told estimates his department would fund Harold's future.

That announcement came with some speculation about a giraffe's gait.

Simon Birmingham smugly declared his research revealed that they amble and can hit up to 55kph.

Sorry, this video has expired How does a giraffe move?

Kabul bomb rocks embassy

For a department with as much on its plate as Foreign Affairs and Trade, the estimates questions easily filled two days.

DFAT Secretary Frances Adamson updated the committee on Wednesday that her staff in the embassy in Kabul were in lockdown because of the enormous bomb blast.

"Our staff there are unharmed, but some shaken. Windows were blown in, even though our embassy was 900 metres away from the site of the detonation of what was a very large bomb," she said.