Treasury secretary Phil Gaetjens has failed to turn up to budget estimates as more junior officials were sent to field questions from senators on Wednesday.
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The no-show raised questions about the senior public servant's absence from one of parliament's headline forums for scrutiny of government spending and decision-making.
Mr Gaetjens, one of the highest paid bureaucrats and among the nation's top-ranking officials, is expected to front senators during Senate estimates questioning about macroeconomic matters as the department's most senior public servant.
Department leaders routinely appear at estimates representing their staff, with the exception of the secretary of the Prime Minister's Department, where there is a convention that deputy secretaries and other senior bureaucrats represent the top public servant.
Treasury deputy secretary, corporate services and business strategy, Matt Flavel, said Mr Gaetjens was interstate for meetings.
One of Mr Gaetjens' deputies, Meghan Quinn, was also absent from hearings on macroeconomic matters while travelling in the US for international meetings.
An unimpressed Jenny McAllister, one of Labor's senators leading questioning in Senate estimates on economic and public administration matters, said Mr Gaetjens had sent a letter on Monday saying he would be unable to attend.
"Could you advise the committee which meetings are more important than appearing at estimates?" Senator McAllister asked officials.
"It's very unusual in fact that the secretary has chosen not to attend the macro discussion, particularly given that the actual deputy secretary is not available to answer questions.
"So we have neither of the officers, the most senior officers in the department familiar with the macro group's work, available to attend in the period scheduled for the discussion of macro."
Senator McAllister called Mr Gaetjens' failure to attend "contemptuous".
Mr Flavel said his boss took his responsibilities seriously and attended another estimates hearing on Friday.
"This is, based on my experience, a more slightly unusual agenda where the department's been returning again for a full day of hearings," he said.
Liberal senator Zed Seselja said there had been no significant changes since Mr Gaetjens' last appearance, and that other officials could answer questions from senators.
Mr Flavel was the most senior Treasury official at the estimates table earlier on Wednesday morning.
The Senate's estimates schedule was set late last year. Senators crammed in a short run of hearings last week while it was uncertain when Prime Minister Scott Morrison would call an election.
When Mr Morrison delayed, senators and officials found themselves back in parliament for another week of estimates. Mr Flavel said Mr Gaetjens did not assume an election would be called at the weekend.