The Motivateme.co.nz address was linked to her website for a short-time before being stopped.
Her apology comes after Ms Elder-Holmes, daughter of late broadcaster Sir Paul Holmes and founder of Facebook page Clean Eatz NZ which documents a healthier way of living, posted on Sunday that she was "super gutted" Ms Lilly had taken the domain names Cleaneats.co.nz and Cleaneatznz.co.nz that she "obviously wanted to use".
When contacted by the Herald yesterday, Ms Elder-Holmes declined to comment further because she did not want "to carry it on".
Ms Lilly told the Herald she had called and apologised to her as soon as she saw her post and told her the domain name was being de-registered.
"I have made a huge mistake and shown poor judgment and I accept full responsibility for this and have apologised."
As a result of the post, she had also received abusive and threatening phone calls and Facebook messages.
All the domain names have now been de-registered.
However, it takes 90 days before the name becomes available again for anyone to register.
A Facebook page owner whose domain name was purchased by Sera Lilly, but did not want to be named, criticised her actions.
"It may be legal, but that doesn't make it right. This country and this industry is far too small to have businesses conducting themselves in such an appalling manner."
The Domain Name Commission spokesman Campbell Gardiner said domain names were available on a first come, first served basis and there was no limit to how many one person could register.
However, if someone objected to a name being registered and felt it rightfully belonged to them then they could apply to the Domain Names' dispute resolution service.