The policeman, who cannot be named, was responsible for coordinating officers arriving at the scene of the mass shooting on March 15, 2019 in which 51 people died.
The inquest earlier heard the first Armed Offenders Squad (AOS) members to enter the Deans Avenue mosque abruptly left after hearing reports of more shots fired at the Linwood Islamic Centre.
They reassured worshippers who survived the Al Noor attack that more help was on the way, but it was another 10 minutes before any other police, along with paramedics, went back inside.
The senior policeman told the Coroners Court today he thought AOS members were in the mosque the whole time, but later learnt that was not the case.
"I found out days and weeks later on. I can't give you an exact timeframe," he said.
The inquest earlier heard one of the AOS members who left Al Noor Mosque believed the officer would find a replacement for his team inside.
But today, the policeman said he was never told to replace the AOS members leaving for Linwood. He said he would have suggested the team stay and reassure the injured until paramedics arrived to treat them.
The officer said there had never been a debrief about the victims being left alone at Al Noor Mosque.
Family lawyer Anne Toohey questioned the officer about the presence of just three AOS members at the mosque once the rest had left for Linwood.
"I've driven home and seen more police at a traffic incident than that. Did this, at the time, seem incredibly thin on the ground to you in terms of a police presence at what must have been New Zealand's worst crime scene?"
The policeman said he was assured by the AOS that there were enough staff to maintain control outside and reassure and monitor victims.
He conceded communication on the day was problematic and information was not relayed as it should have been.
The senior officer, who had been with police more than 30 years, said he had not had training for a mass casualty event.
