Updated 5.12pm: National's promising $200 million more for the ultra-fast broadband rollout - but it's not yet clear if Chorus will be in charge of the programme's extension.

If re-elected, John Key says National will increase the rollout's coverage from 75% to 80% of the country.

He's made the announcement at Hamilton's Wintec institute, with more details expected shortly.

It's not yet clear whether Chorus will be given responsibility for the extension, amid concerns about its ability to afford the remainder of the existing rollout.

While it’s not guaranteed the extra work, National's Amy Adams says it’s still a possibility.

"Well they are certainly welcome to pitch. I imagine they may well want to but they will have to work through the process like everybody else."

40% of the fibre for the current scheme has been rolled out, and Amy Adams says consumer demand is expected to catch up.

She says uptake is currently at 8% but that's ahead of expectations for this stage of the rollout.