Got the tip from someone and a confirmation from Vodafone: from 25 january 2006 Prepay customers will see an increase in the charges on SMS for international destinations.
According to Vodafone, the effect only applies to PrePay customers, due to different pricing strategies for PrePay and On Account customers.
The official word is "The effect only applies to PrePay customers since we have different pricing strategies for PrePay and On Account customers. In order to keep some of our core services at their current price it means we sometimes need to increase the price of other services - at this time, International TXT. It will still be free to receive international TXT messages. FREE TXT weekends is for National TXT messages only – ie. TXTs sent within NZ between 021 and 029 mobiles."
I agree that prices fluctuate, and if this means some other services can be kept at current levels or lowered, then that's an ok move.
We have to see how this unfold in terms of market. I think Vodafone wouldn't do this if international SMS ("Vodafone TXT" in their lingo) was a big chunk of their business.