If I top up my Pingit account with my debit card, are my purchases protected under the card's chargeback scheme?

No, because the things you buy with Pingit aren’t classed as card transactions. That means you can’t ask for the transactions to be reversed or refunded under the Visa or Mastercard chargeback scheme. 

You’ll need to get in touch with the seller directly if you have problems with what you’ve bought.