Porting of mobile phone number at end of On Account contract with Vodafone

Hi all,

I have looked around the internet and this site and can't find a clear answer as to what I should do to port my number, without losing a months credit on the account.

Currently I have a 24 month On Account contract with Vodafone which costs $39.99 a month (was $39 until it was increased mid-term a few months back). This contract expires on 28 May 2016.

As the plan only offers 300 nz & aus mins, unlimited txts & 750mb data (I have asked Vodafone on several occasions to increase this to 1GB or 1.25GB to reflect the latest offerings on their website - they always said they would, and never did - yet happily increase the bill by $.99 for the tele fee), I am going to go with skinny or 2degrees who offer 300/1gb/unlimited for $26 & $29.95 respectively. Up to $14 cheaper than Vodafone per month for the same deal. 

As my contract finishes/expires on 28 May 2016, when should I look to port my mobile number, without having to incur an additional 30 days cost on the account (for credit I will lose)? Should I wait until 28 May 2016 for the new bill to come through for 28/5/16 to 27/6/16, and port the number then? But Vodafone require 30 days notice from me? Or should I contact Skinny or 2degrees to port the number in advance? (is this possible)?

Help would be appreciated! Cheers