Labor frontbenchers have claimed “labour shortages” and “market failures” are behind Bill Shorten’s speechwriter being given a $300,000-a-year contract.

On Monday afternoon, it was revealed in Senate estimates that Julianne Stewart, a former speechwriter to four prime ministers, had been hired in September 2022 for $620,000 for a two-year period to write speeches for Shorten’s office and mentor staff in Services Australia. Stewart has been paid $447,516, excluding GST, so far.

But on Tuesday, Labor frontbenchers jumped to Shorten’s defence, with ministers Tony Burke and Chris Bowen explaining the unusually high offering was the result of “labour shortages” while accepting the role would “absolutely not” be paid that highly under regular circumstances.

Services Australia officials had told senators the agency had about 200 media and communication staff but they lacked a professional speechwriter after another in the role had retired.

Veterans’ affairs minister, Matt Keogh, said he was “pretty convinced” Shorten still wrote his own “zingers” – a reference to the former Labor opposition leader’s sometimes sharp remarks.

The deputy chief executive officer, Susie Smith, explained on Monday a “market failure” meant an external contractor was needed for the role and that “to remain competitive [you] need to pay a market rate”.

Shorten denied any involvement in the negotiation of Stewart’s contract on A Current Affairs, but said she “does a very good job”.

Stewart makes almost double some of the highest-paid speechwriters within the agency, which confirmed those among its senior ranks would take home about $140,000 a year.

Guardian Australia sighted a number of recent job ads within the public service for speechwriter roles.

One within the Department of the Prime Minister and Cabinet advertised in October 2022 – a month after Stewart started the role – offered a salary between $113,627 and $129,502 for an executive level 1 role. Another job ad at the infrastructure department for a “senior speechwriter” offered between $113,631 and $119,891. Both roles required creating speeches for ministers within the department.

The salary for an executive level 1 and executive level 2 role, which Stewart had likely been hired under, was between $109,738 and $151,019, according to Services Australia’s annual report in 2022-23.

The estimates revelation drew criticism from the opposition, including from shadow treasurer Angus Taylor, who accused Shorten of wasting spending.

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Burke told ABC radio the country’s ongoing labour shortage meant “various professions are getting more money than [they] would otherwise be paid”.

“Would that job ordinarily be paid that amount? Absolutely not. For the exact reasons that I’ve explained, that’s why it landed in the place that it did,” he said.

Bowen also told ABC radio a large media unit is useful because the millions of people who talk to Services Australia “need things explained to them”.

“I mean the number of people who talk to Services Australia about their issues and need things explained to them ‑ it literally runs for millions of people, and speeches and communications from that agency, Services Australia, are pretty damn important in communicating with those people who have questions about how Australia’s social security system works, and the impact on them,” he said.

First-time Labor MP Sally Sitou told ABC she would not have paid that much for a speechwriter.

“Would I have paid that amount for a speechwriter? No. But we’re in a tight job market and this is the outcome.”