yitz: Will be interesting to see any developments this year... I wouldn't be surprised if OP receives notification that the legacy service/plan will be discontinued sometime this year... however the copper withdrawal code does state that "before the copper service can be withdrawn an equivalent fibre service is readily available at no additional cost to the consumer". I wonder if Vocus group has kept the Flip brand around to battle this one out in regard to seeking lower wholesale fibre costs.
https://comcom.govt.nz/regulated-industries/telecommunications/regulated-services/consumer-protections-for-copper-withdrawal/copper-withdrawal-code/timeline/commission-launches-consultations-on-safeguards-for-consumers-moving-from-copper-to-fibre-broadband
The actual copper service itself is not being withdrawn in the above example. As the OP could just switch to Spark or any other ISP that resells Chorus Baseband Voice, if they want to keep a copper landline connected. (for now as least) And they are in a UFB area. So fibre is readily available and can be installed for free.