These are details for Liz, but she isn't still in charge of Potty Babies Wellington. She is however still around. I'm not sure if mail gets forwarded to Rachel, who I think is the contactperson now. If you want I can PM you some contact details, unless you've already found them! (Or e-mail you, if e-mail is still same as on your web-page?)
Easiest way would be going to the Homebirth Association's next coffeemorning, second Thursday of every month 10-12 at Trinity Union Church, but you knew that! You should be able to find playgroups, music groups, Steiner playgroup, potty group, natural or alternative mum's groups etc, through the mums there. Unfortunately, the coffeemorning would have been this past Thursday.
I'd say word of mouth is the way in Wellington, it took me years to find things there, and when I did it was actually thanks to someone posting on Mothering!