An eager spender who blew more than $100,000 using other people's credit card accounts has prompted an overhaul of Westpac's online application process.

Taranaki's Grainge Madgwick has promised to repay the debt he incurred after being given credit cards on accounts held by up to 10 other people. Police are investigating him after the Westpac credit card fraud investigation department laid a complaint. Westpac investigators state Madgwick used the details of others to get credit cards.

Changes to the application process were made after cards were issued to Madgwick on a number of accounts opened online in the names of other people, mainly family members. Madgwick was listed on the credit card application forms as the "additional cardholder". Under bank rules at the time, any spending on the credit card was charged to the "primary card holder".

While it was unclear who filled in the application form, one account holder said he was astounded to be called by a collection agency demand-ing $22,000 for debt on a Westpac credit card.

He said it took days to convince the bank he knew nothing of the spending which took place in August 2009 and reached the limit of $15,000. The other $7000 had accrued in interest and late fees.

Westpac later provided account details and the application form, showing Madgwick's name as the additional card holder.

The application details for the man carried his name and date of birth but the remaining details were bogus.

The account statements included an August 3 withdrawal of $1400 that astonished the man. He said it was his birthday and he had met Madgwick to celebrate.

"He was buying me beers for my birthday."

The man said he had been paid $3000 by Westpac as compensation but had laid complaints with the Commerce Commission and the Banking Ombudsman.

Madgwick said: "I'm not going to confirm or deny anything. This debt is my debt. The arrangement I have with Westpac is this debt is my debt. These other people have zero involvement with it."

In a later interview, Madgwick admitted he had a credit card as an "additional card holder" on the account of the man interviewed by the Herald on Sunday.

However, he said that the card holder knew about the card application and his use of the card - a claim the upset account holder denied.

Westpac spokesman Chris Mirams said the process for online credit card applications had been reviewed: "We have more robust measures in place, particularly around additional card holders. We have made a complaint to police about the person."