Hi Georgie, I found your post after googling Opawa Playgroups.  We're moving from Wellington to Christchurch next week and are also going to be living in Opawa.  I'll let you know if I come across any playgroups that look good, and it'd be great if you could do the same? 
 
 
I also came across the Opawa St Martins Toy Library which looks good, but it hasn't reopened yet since the earthquakes.