I co-ordinate a World Vision donations group at work consisting of about 20 people's donations. I apply for the charitable rebate on everyone's behalf. It went sooo easy when I recently applied for the rebate for 1 Apr 08 - 31 Mar 09 tax year.
World Vision sent me the receipt within a week. I filled in the appropriate IRD form and found the money in my bank account within the week even before the tax summary arrived in the post! 
If you need any other advice in this area just PM me - I'm quite familiar with it, or just go to www.ird.govt.nz and there's a section in there on charitable rebates with lots of helpful stuff.