The watchdog has been told Mr Manning leaned on highly paid consultants as his agency oversaw a book of projects worth more than $6 billion.
Former Investment NSW boss Amy Brown faced the corruption probe on Monday and denied she was ever close to Mr Manning.
The pair had both worked on the controversial Northern Beaches Hospital public-private deal before she reviewed his CV and acted as a "sounding board" when he sought to lead the new school infrastructure agency.
But Ms Brown denied helping him land the chief executive role.
Her advice was limited to cosmetic changes to a CV and helping him prepare for a job interview, she told the inquiry.
"I was never friends with (Mr Manning)," she said.
But she recalled making a passing comment about Mr Manning having the skills to lead a proposed infrastructure delivery unit in a February 2017 meeting with former education minister Rob Stokes.
School Infrastructure NSW was established in April 2017.
"I would have said the education department needed a specialist unit ... I would not have said they needed Mr Manning," she said.
Ms Brown was working as a consultant at management company PwC at the time and was sharing her thoughts about potential delivery models for school infrastructure.
She was later sacked from a departmental secretary role in 2022 after a review found she was "indirectly influenced" by then-trade minister Stuart Ayres in the appointment of former deputy premier John Barilaro to a lucrative overseas trade job.
The corruption watchdog later found no evidence of corrupt conduct.
After being appointed School Infrastructure chief executive, Mr Manning awarded contracts worth millions to PwC and another firm, which has been cited as an early example of an alleged "broader pattern of behaviour" to depart from proper procurement processes.
All up, at least $344 million was spent on independent contractors before Mr Manning left in early 2024.
Education secretary Murat Dizdar has told the commission he was concerned about contractors making up almost half of the school infrastructure agency's employees.
Twenty-six contractors were earning salaries similar to his own, which exceeded $500,000.
"I saw that as a massive imbalance, and I wanted to correct that," Mr Dizdar told the inquiry.
But he struggled to achieve that under Mr Manning, who claimed "there were roles ... that could not be achieved" by permanent staff due to public service pay rates, Mr Dizdar said.
While not commenting specifically on the commission's investigation, Education Minister Prue Car told reporters on Monday Labor came to power in 2023 aware that "for some reason", not enough schools were being built.
"We did have some serious concerns about school infrastructure under its previous leadership," she said.
Mr Manning is expected to be among the final witnesses called during the six-week public hearings.
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