If you are willing to learn the basics about double-entry bookkeeping, then it’s entirely feasible to keep your own books until it’s more valuable for you to pay someone else to do the work (and even then, a bookkeeper will likely spend a bunch of time chasing you down for receipts, categorizations, etc).
Platforms like Xero have guides on bookkeeping for small business owners that can help you get started.
It’s probably worth choosing a platform like Xero or MYOB for their NZ-centricity and ability to connect to NZ banks accounts, account for GST, etc. If you are really strapped for cash, there are free options like Wave or various spreadsheet templates. However, the time you save with something tuned for the NZ market is likely worth it.
Whatever you choose, even as a sole trader, make sure you understand when and how you need to charge, collect and write off GST and what your tax obligations are. (E.g. after your first year you will need to pay provisional income tax.)