This currently happens in the US - so not unlikely, but hopefully not :)
It also happens in Australia. Seven black out (and by black out, I mean show endless reruns of Border Security) all domestic cricket and AFL as the do not have online rights (owned by Foxtel and Telstra respectively). Nine also had an NRL blackout until last year. It will all depend on the terms of the agreement between Spark, TVNZ and the RWC rights holder.