Spark says $33 refund won't be automatically applied as it wants to give customers the choice of claiming an upgrade or making a donation.

Hundreds of thousands of Spark customers will get a $33 credit on their ​phone or internet bill – but only if they go online to claim it by the end of April.

The refund is designed to honour a commitment the company made last year and applies to people who were on capped copper-broadband plans between February and November, or who had a phone-only plan in Auckland, Wellington or Christchurch.

Customers still on capped plans will have the alternative of choosing a free six-month upgrade to their broadband plan, worth from $60, or Spark is giving customers the option of donating the $33 rebate to charity ​through ​givealittle.co.nz.

But spokeswoman Vicky Shepherd said that if customers didn't choose one of the options by April 30, they would miss out, as the refund would not be applied to people's phone bills automatically.

READ MORE: Spark raises prices $5 a month, but confirms refund for earlier rise

Spark would remind customers of their entitlement on their March and April phone bills, she said.

A Commerce Commission spokesman said the way the refund scheme worked was "a matter between Spark and its customers".

WHY THE REFUND?

The refund has come about because the Commerce Commission did not follow-through on its idea of letting network company Chorus backdate higher wholesale fees that it charges Spark and other phone companies for access to its copper network.

Spark, along with most other internet providers, raised prices for most of its customers by about $4 a month in February last year, in its case explicitly because of the backdating threat.

It was alone in promising customers a refund if higher wholesale prices weren't backdated.

The Commerce Commission confirmed even higher Chorus charges in December, leading internet providers to again raise their prices for home phone and broadband connections, by about another $5 a month.

But it decided against backdating the price rise, triggering Spark's refund commitment.

HOLIER THAN THOU?

Spark manager Jason Paris said the company was doing "the right thing" by refunding customers for the extra they paid last year.

"Most broadband retailers increased their prices around the same time [and] to our knowledge we are the only retailer who is now giving back to customers," he said.

Major rival Vodafone denied a clear link between the backdating threat and a price rise it imposed last year. It never promised a refund and has indicated one won't be forthcoming.

- To claim the $33 ​phone or internet credit, click here.