Europe has GDPR, so you're not legally allowed to collect data unless it's necessary for the actual service you're providing your customer, and you're not allowed to use data for anything else once you've collected it for the purpose you stated.

Having said that, your customer will always have to prove who they are, how they acquired funds, and where funds are going. This is to prevent bribery & corruption, money laundering, terrorist financing, tax evasion etc.

I was working as a software developer for an EU investment bank when the EU implemented GDPR, and the amount of paperwork required to collect and hold personal data meant we destroyed a ton of data & documentation and rewrote a lot of software. And every spreadsheet containing personal data or which was used more than once had to be recorded in an EUC register with signed commitments about GDPR compliance. Even if data wasn't strictly forbidden by GDPR we'd be very wary asking for any information which could theoretically be misused to discriminate against protected classes.

The NZ Privacy Act 2020 looks broadly similar in intent to the GDPR, so I imagine there'd be the same disinclination to collect information which can't be proven necessary to perform the requested service or satisfy regulatory requirements.

I have several NZ insurance policies and they had no interest in transaction history. Same with my mortgage. I sent bank statements, but only as proof of address.

Only my credit card application wanted to drill into my spending, which is not unexpected considering it's unsecured lending. For sure I'd rather approve the API access than try to find where I can download (& probably pay extortionate fees) for copies of historic statements