Prime Minister John Key said tax changes to help business will cost hundreds of millions of dollars.

Parliament will sit under urgency so the tax changes -- to be announced tomorrow -- can come into effect by April 1.

Today Mr Key was asked about the fiscal impact of the changes.

"It's reasonable, it's not insignificant. You'll see the number tomorrow but it's not in the billions."

Asked if it was in the hundreds of millions, he said yes.

Parliament resumes for the year next Tuesday and will sit under urgency on Wednesday and Thursday. The Government would allow question time to go ahead despite the extended sitting hours.

Other legislation will also be debated under urgency.

Mr Key is to give a speech in Auckland tomorrow outlining the Government's plans to help small and medium businesses weather the rough economic climate.

There would be a range of initiatives aimed at taking pressure off the sector.

Changes would focus on tax, cash flow and regulation.

Changes are expected to include reduced penalties for companies which underpaid provisional tax.

The tax is paid in advance based on estimated earnings. Underpayers get hit with a 14.24 percent penalty.

Another likely change was to remove the presumption that profits would increase 5 percent on the previous year for calculating provisional tax.

Other legislation likely to be debated under urgency was Resource Management Act reform and a number of law and order measures.