Simon Bridges says the previous Government had no idea about the state of Middlemore Hospital, placing the blame on the district health board.
Several buildings at the hospital, located in south Auckland, have serious rot and mould problems, posing health risks to staff and patients.
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"We didn't know," the National Party leader told The AM Show on Monday.
"I've checked that [former Health Minister] Jonathan Coleman and with others as well. We just didn't know."
He said the Manukau DHB was requesting extra money, but no one mentioned why.
"The board should have been telling us about these things, and they weren't. They should have been telling us about what was going on there."
Marama Davidson, new co-leader of the Greens, says she gave birth at Middlemore and its awful state is absolutely National's fault.
"When the previous National Government were focused so hard on a surplus at all costs, yeah they're probably going to ignore and not proactively make sure that the hospitals are doing okay," she told The AM Show.
"It's not good enough that Simon Bridges has just finished telling you 'we didn't know' and that's his fob-off. That's not good enough at all. They needed to ensure they knew what was going on."
Mr Bridges suggested the scale of the problem was being exaggerated by the present Government to "create a sense of worry" and it should "stop whining".
"In the scheme of things, the money for this won't be big... they should just get on and fix it because we've left them a strong economic legacy to do so."
Mr Bridges said National increased health spending every year when it was in power. But those boosts sometimes fell short of matching inflation and record population growth, according to a Ministry of Health briefing.