Acting Prime Minister Winston Peters is defending Corrections Minister Kelvin Davis, saying he is completely across his portfolio.
That comes after Mr Davis told Morning Report on Monday that he "hadn't seen the forecasts" when he last month announced the decision to build a 600-bed facility at Waikeria instead of a much bigger one.
He later did a swift backtrack, saying he was aware of the latest prison population projections when the government made the Waikeria Prison decision.
New Zealand's prison population is being forecast to rise by more than 4000 over the next decade, according to a Ministry of Justice report. That would take the number of inmates to 14,400 by 2027.
The number of prisoners being held on remand - those awaiting trial or being held prior to sentencing - was predicted to almost double over that period to 5400. The report said that was because of an increase in the length of time people were spending waiting for their case to go to trial.
Mr Peters said Mr Davis thought he was being asked about the full report, not the specific forecast numbers.
"Looking back, I can see how someone can make that mistake," he said.
"If you're talking about the telephone book versus the Yellow Page directory, you can be talking about two different things - if you're looking for the same phone number, so to speak," he said.
Mr Davis was completely across his portfolio, he said.
"Is he aware of the elements of penal reform and custodial numbers that we are referring to as government? Yes, he is."
Earlier, National leader Simon Bridges said the "perplexing statements" showed Mr Davis wasn't across his portfolio.
"He's either not telling the truth or he's simply not up to the job - and sadly, I think the latter is probably the case," he said.
The government had to explain its plan in light of the fresh forecasts, he said.
"Either they have to build more prison beds to keep New Zealanders safe - or they have to soften up the bail, sentencing and parole laws."