After signing petitions to save the Lumsden Maternity Centre and an Auckland-to-Kapiti flight route, some signatories received emails from the party asking for donations.
Mr Walker, who started the Lumsden petition, said the emails were the result of a "genuine cock-up''.
"As soon as I was made aware of the error, I ensured we sent out an email to apologise, take the affected parties off that main distribution list and ensure it doesn't happen again.
"You work your backside off to retain a vital service in the community like this, and whenever you have humans involved, you always have some error.''
A statement from an unnamed National Party spokesman did not say how many received the emails, but said their information was added to its main email distribution list after a "small error in two of our petitions''.
"We have since gone back to each individual concerned, apologised for the error, and removed them from the main email list.
"This was the result of human error when building the petition. The process for doing so has now been fixed to ensure it won't happen again.''
Mr Walker said more than 4000 people had signed his petition to oppose the downgrade of Lumsden Maternity Centre from a primary birthing unit to an antenatal centre.
"That's a remarkable result and goes to show how much support there is from the community that we do everything possible to keep this service.''
The petition went live on March 8.
