The entire prison site also has the main perimeter fence and gate system - which includes a vehicle checkpoint.
One source said Corrections officers coming to work each day they are supposed to leave their belongings in staff lockers near the prison's main entrance.
By the time they get to the unit where they work they should only be carrying their lunch, which is stored in a staff area there.
"But as the prison has expanded, there are not enough staff facilities," the source said.
"So staff [in Willis' unit] didn't have lockers outside it and were bringing their stuff inside.
"Their gear sits in the staff area there all shift."
The source said Willis was tasked with cleaning the staff area each day and simply stole a prison guard uniform.
It was not clear if he had been "collecting bits of uniform" for some time or "just grabbed it that day".
After Willis allegedly donned the guard's uniform, he put phase two of his escape plan into motion.
Major construction company Naylor Love is expanding Rimutaka Prison for Corrections and it is understood one of their vans and a number of workers were in Willis' unit.
"To avoid compromising the police investigation, prejudicing the criminal proceedings, or impacting the reviews under way, we are limited in what information we can provide.
"In addition, we are unable to provide detail on our specific security measures to avoid further risk to the safety of the public, our staff and prisoners."
Police also refused to be drawn on the specifics of Willis' alleged escape.
"A man is remanded in custody and due to appear in Masterton District Court on 1 March charged with burglary, theft of a motor vehicle, and escaping custody," said a spokeswoman.
"Police investigations continue."
Earlier, Corrections acknowledged the serious nature of the alleged escape.
"We clearly have very serious concerns for how this escape was able to occur," it said.
"We have made some immediate changes to security and operations at the prison, including pausing building works being undertaken.
"We appreciate that New Zealanders rely on us to securely manage the people directed to reside in our prisons, and they will be rightly concerned about this incident.
"We assure the community that we are absolutely committed to preventing a similar event from ever occurring again."
They said a security practice review was under way and an operational review by the chief custodial officer and an independent reviewer who is yet to be appointed.
"We identify failings, they will be remedied with urgency," Corrections said.
Corrections Minister Kelvin Davis was informed of the escape as soon as Corrections became aware and was regularly updated on the search for Willis.
"I understand some immediate changes have been made at Rimutaka and I would expect a thorough review to be completed to minimise the risk of this ever happening again," he said.