I'll preface this with the fact that I am no longer a customer of 2 Degrees. 

I moved to One NZ on the 1st of November. They offered a similar package but can provide cellular Apple Watch connection, so I jumped ship. 
Shortly after I received a bill from 2 Degrees for the month of services I had just received. I duly paid it. 
I was expecting a pro-rata bill for the few days I continued to use 2Degrees while the Number port was completed. 

I was immediately cut off from all 2D services - including the app and website, so I couldn't access bills, charges or anything to do with my account from the moment the port was successful. That was 1 November. 

My Billing profile has a billing period from 26th (month) to 25th (next month). I was on the $60 a month plan. 

I've just received a bill for the period 26th Oct - 25th Nov. For the full amount of the plan! A plan I have not used or had the privilege of accessing. 

Fast forward to my phone call to the customer service desk. "You're on Post-paid. You owe a month's bill". No - I owe you 5 days of usage, which equates to $10.00. 
Lots of back and forth where the CSR argued that I was wrong and that I would have been told when I went to another provider that I'll be on the hook for a whole month. I never spoke to 2D at any stage. Nobody called to try and prevent me leaving. No emails, no texts, nothing. 
I asked to speak to a supervisor. I was put on hold and then the same lady came back and said I can get a credit for $30.00 and that this was special and intimated i should be grateful. 
I refused the offer and again asked to speak to someone more senior. I got that person, who was rude, officious and basically said that I should know I was on the hook for a month's charges and that the offer was the best i was going to get. 

So - I will be paying $30 for services I have not received, despite my insistence that this is effectively theft or fraud. 

Suffice to say, I will never go back to 2D or ever encourage anyone to go near them. I'd rather pay more somewhere else that ever deal with this company again. I was once a massive advocate. Never again. 

TL;DR - Left 2D mobile. Had 5 days' services worth $10 and was charged $60.00. After arguing with Customer Services I was offered a $30.00 credit and will be required to pay 3x what I should pay. I'll never go back to 2 Degrees.