It depends...

If the 2Degrees connection has been disconnected the number would go back to One NZ and be quarantined. If the number has not been re-allocated it can be re-issued. 
A number of factors need to line up for this to happen.
1. Front line call center staff often don't have the ability to change a number, your best bet is sales signing up a new on-account connection. (They can search available numbers when setting up a new connection however I don't believe these include quarantined numbers).
2. Unless things have changed, Prepay customers do not get the option of choosing a number, The most you can do is look through the sims that the store has to pick a number you like as they are all pre-allocated to prepay sims.

@Sealc Have you called the number and made sure it is not allocated to another SIM / end user??

What type of number was it originally when it was with One NZ, Prepay or On Account? What were the first 3 digits of the number after the 021? (please don't provide the whole number)
Prepay numbers generally start with 021 0XX XXXX(X), 021 1XX XXXX(X)
On Account numbers generally start with 021 3XX XXX, 021 4XX XXX, 021 5XX XXX, 021 6XX XXX, 021 7XX XXX, 021 8XX XXX, 021 9XX XXX (and some parts of 021 2XX XXXX)

TelstraClear numbers 029 XXX XXXX - From memory these numbers aren't reactivated anymore as they sit in the old TelstraClear side of the system that isn't actively used.

If it was a Prepay number, it would be near impossible to get it back after 10 years, as once the number is disconnected and returned to One NZ, it sits in quarantine for 3 months, then goes back into the pool of prepay numbers to be created for prepay packs and sent out to stores for sale. (this cycle will continue each time the prepay number is disconnected)

If it was an On Account number, after disconnected and returned to One NZ, it sits in quarantine for 3 months, then goes back into the pool of available numbers for new On Account connections. From there either:

• the number would be available for a new On Account connection, or;
• within the last 10 years the number has been used for another person and active on the One NZ Network, or subsequently ported out to another mobile provider and active on their network, or;
• in rare occasions the number could be blocked from being re-used (either deliberately or due to an error in the system) - This would need a ticket raised for investigation, and if allowed be made available for a new On Account connection.

Moving the number around from On Account to Prepay shouldn't make a difference, as the rules follow what the number range was originally designed for. (Rules that have existed before the ability to move between On Account and Prepay).

From memory that number range 021 8XX XXX specifically did have some restrictions in the past, so the number could be locked. Another reason it might not be available is if the number was used by someone else but now recently disconnected. Due to privacy reasons, you will not be told why you can't have the number. (annoying but privacy is important)

Best option is to visit a retail store and ask them if it's possible to get the number for an On Account connection. They can see the status of the number and if they think it is possible to get the number, they can get a technical ticket logged.

Number was ported out to Spark in 2014 and returned back to one.nz in 2022 and doesn't appear to be assigned, so it should be available.

Sealc: Thanks for letting me know. So I can just go to any one nz and get the number ?

@Sealc Yes on Postpay only then request a payment flex to Prepay if you want

The store can explain what a payment flex is and how it is done but basically moving a number between Postpaid & Prepaid