National has announced an extra $120 million to build a new Christchurch stadium - and wants the arena to have a fully-closed roof.

The new funding pledge was announced by Prime Minister Bill English in Christchurch today, and will take the Government contribution to $179m.

The preference is to have a 30,000-capacity arena with a closed roof, but the end design will ultimately be up to the local community and council.

The Government said today's pledge meant there was enough money for a 30,000-capacity stadium with an open roof - and Christchurch ratepayers will need to find more funding if they opt for the more expensive closed-roof option.

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Christchurch Mayor Lianne Dalziel has ruled out putting up more ratepayer money into the project or selling assets to help fund it.

Labour Party leader Jacinda Ardern will be in Christchurch on Sunday and make her own announcements regarding the earthquake rebuild projects.

English said he had been advised the shortfall after today's funding announcement was about $80m.

"That will largely be a matter for the council and looking at the way other stadiums have been funded. So that's often a combination of sponsorship or other forms of local funding. So that is a discussion that can now be entered into."

English said a stadium with a full roof would allow multi-use of the facility.

Greater Christchurch Regeneration spokeswoman Nicky Wagner said the timeframe talked about for the stadium build had been 2020, but that could be brought forward.

Today's announcement was a party one - meaning the funding would be made available if National is re-elected.