Security Error: Domain Name Mismatch

A site I have been working on now gives this message every time going from http to https (secure connection.) The problem is this puts customers off buying on our site even though https is working the difference is one has a www. prefix while the other doesn't. Is there a simple way round this with regards to use of apache or something? Its a very annoying message and I would hate to have to get another certificate because of this.
Any help would be greatly appreciated.

Please post the exact text of the message. What are the HTTP and HTTPS domains? What domain is used in the certificate?

You have attempted to establish a connection with "soccerscene.co.nz". However, the security certificate presented belongs to "www.soccerscene.co.nz". It is possible, though unlikely, that someone may be trying to intercept your communication with this web site.

Wouldn't it be possible to prepend the "www." when the switch to HTTPS is made? After all, the application already knows when the switch happens (since it generates HTTPS URLs instead of HTTP ones).