The Finance Minister says she'll confirm in the Budget how much extra the Government will have to spend - although she's still promising tax cuts in July.

Nicola Willis broke convention today when delivering the Budget Policy Statement by not detailing the operating allowance.

She says the number will be lower than $3.5 billion dollars previously forecast.

But Willis says she's not fazed to be straying from the norm.

"Grant Robertson has a practise where he announced the number six months out, and then he'd deliver a completely different number on Budget Day. And I don't think that's particularly helpful, I want to be more reliable than that."

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