"The government passed the tougher bail laws, those are being implemented I think probably even more rigorously than we expected," he said.
He said the government could not reduce the number of people on remand but could do more with those people while they were there.
"A whole range" of things would be needed to reduce numbers of people in prison, and those had to be done in the context of public safety.
Upston told The Nation's Lisa Owen there was no need to review bail laws.
"In terms of bail legislation and sentencing, it's absolutely spot on," she said.
Upston said National had an "unrelenting focus" on reducing reoffending.
In the last year, Corrections had supported 2000 prisoners into jobs once they left custody, she said.
Davis said New Zealand was heading towards having 18,000 people in prison.
"We have to have a paradigm shift in the way Corrections is run."