"We had a city awash with buildings we had not searched and this huge list of missing persons. At what point do you say, enough is enough?" he said at the hearing in Christchurch today.
"I was conscious that we had the greatest number of personnel working at the CTV site and were committing resources at the expense of multiple sites across the city.
"It's horrific, but if you don't make these logical decisions, you are overcome by emotions. It is tough, thankless, and you wouldn't wish it on anyone."
Several victims trapped beneath the debris had been in cellphone communication with loved ones, but rescuers had noted no signs of life since about 1.30am on February 23.
Meanwhile, Mr Stuart-Black had heard reports of "multiple people" trapped in the crippled Christ Church Cathedral, and knew of "numerous other locations" that had not been checked by USAR.
At 11.15am on February 23, he ordered USAR rescuers off the Madras St CTV site, to redirect their energies and skills at other sites.
He knew his decision to abandon the site - where 115 would die - angered his workers.
And knowing it would attract "significant attention", Mr Stuart-Black took it upon himself to tell the top fire chiefs.
But soon after, he sent staff back to the site to continue the search.
They stayed until the site was clear, dragging out bodies which was not a USAR job, but as he put it: "There was no one else to do it."
Earlier, he told the inquest no country in the world would have had the rescue resources to cope immediately with the CTV disaster.
Even if the total collapse of the six-storey Christchurch office block was the only emergency site after the magnitude-6.3 quake, the emergency services would have struggled to cope, Mr Stuart-Black says.
They had to deploy the country's three USAR taskforces while sending out an SOS call to international comrades to try to tackle the immense disaster.
Plans had to be "augmented and supplemented", he said, as they scrambled to find survivors.
"There are very, very few countries in the world that have enough capacity that they can meet the need, in short order, of something that was as big as Christchurch," said the national manager special operations for the New Zealand Fire Service, which includes responsibility for USAR.
"And I would suggest that no country could've hit Christchurch's response in the first 12- 24 hours with sufficient resource."
USAR is a crack unit that specialises in finding and recovering people in dangerous situations.
NZ USAR taskforces are based in Auckland, Palmerston North and Christchurch, and made up of firefighters, dog handlers, communications experts, engineers, doctors and paramedics.
The Christchurch experience has provided several lessons for the international USAR community to learn from and will be incorporated into future guidelines.
The coroner's probe is looking into the deaths of Tamara Cvetanova of Serbia, Cheng Mai of China, Japan's Rika Hyuga and Jessie Redouble, Emmabelle Anoba, Ezra Medalle, Reah Sumalpong and Mary Amantillo, all from the Philippines.
All were students at King's Education School for English Language on the CTV Building's third floor and survived the collapse but could not be rescued from the wreckage.
The inquest, before Coroner Gordon Matenga, continues on Monday.
