My advice is not to register until you need to. Lawn mowing is labour-intensive, so you will always pay more GST than you claim.
Am I not better off claiming that back?
The short answer is No. As the GST threshold is currently $60K, you would need to spend over 60k on start-up equipment to make it worthwhile.
My advice is not to register when you start and aim to get a business going that turns over $1000 a week BEFORE deciding if you want to build the business any bigger.
You do not need to register if your turnover was less than 60k in the last twelve months.
You must register if you expect your turnover to exceed 60k in the next twelve months.
You must also register if you are currently charging GST to your customers. If you worked with mainly commercial premises, for example.
Most lawn mowing businesses do not charge extra for GST at the beginning; they simply absorb the cost of the GST when they need to register.
Here is a worksheet from IRD that you can use to decide if you want to register now or later.
Do you need to register for GST.pdf
https://s3-us-west-2.amazonaws.com/secure.notion-static.com/0e81c18b-3833-4fa3-937f-938ecacd73e3/Do_you_need_to_register_for_GST.pdf
An NZBN is not required, but it can be helpful. When COVID happened in 2019, the government wanted NZBN numbers before any payments were made. Also, Google has been known to ask for them as proof of ownership of a business.