He said there was no specific fund to tap into but the Government had contributed to similar projects in the past.
"We always want to see that the local community really want it and in a way that's really demonstrated by their level of financial commitment," Mr Key told the Chronicle.
"Given that you've already got half the facility here really the question is are we all prepared to chip in a bit more and do the second bit of it."
Local fundraising for the $13million project is already underway and the idea of a $4million taxpayer contribution was casually floated by the roof's supporters at yesterday's function.
Mr Key said it would be considered in Wellington.
"It's not like they're asking for a ridiculous sum of money that would be too difficult for the Government to get to," Mr Key said.
"But like everything there's always lots of demands on us and a limited amount of cash so we just need to work our way through it.
"It's within the ball park of things I've seen around the country we've done before."
Bike NZ CEO Andrew Matheson said the organisation was in the process of setting up regional performance hubs around the country to support the national centre in Cambridge and Whanganui was part of that plan.
"We are fully behind this project," Mr Matheson said. "We are committed. We're going to put a fulltime coach based around this area."
Mr Key supported that idea.
"We always looking to see what sort of benefits will come if we make an investment which is somewhat out of the ordinary," he said.
"I think there's a fairly logical coherent story there around the sporting side and the development of cycling and I think they have got quite a fair point that it won't all be in the national facility which is in Cambridge," he said.
"I do like it and it will definitely work."
Meanwhile, soil capacity testing has been done on the site with bore hole testing to be done next.
"The ground conditions are very good actually, much better than we anticipated," Bob Smith of the Raise the Velo Roof team said.
The team have the first members of the 5000 club signed up as financial sponsors as well as an individual contribution of a "substantial sum".
The team are also looking for a naming rights sponsor and will launch further initiatives in the coming months.