One email came through today with this URL to the new post: http://www.geekzone.co.nz/forums.asp?forumid=97&topicid=83026&page_no=195#588710. At the time the link came though, the thread was only up to page 194.
If I modify the page number in the URL to be 194 (http://www.geekzone.co.nz/forums.asp?forumid=97&topicid=83026&page_no=194#588710) then the link takes me to the correct post.
When you click on a link with an invalid page number, you get taken to the thread but see no posts. In order to get to the new posts, you need to click on one of the correct page number links.
I don't know if this problem affects all topics, only topics with large numbers of pages, or just this one topic. I'll try subscribing to a few threads and see what URLs come through.