How can I buy fertiliser and have a guarantee that I'm actually getting what I'm paying for?

One way of doing this is to select products that are Fertmark registered. Here is an explanation of the Fertmark programme on the NZ Fertiliser Association website:
The Fertmark programme was established in 1992 to give New Zealand farmers confidence in the quality of fertilisers and associated advertising.

It now includes the requirements of the Agricultural Compounds and Veterinary Medicines Act that fertilisers are supplied in such a way as to minimise risks to food safety and animal welfare.

The Fertmark Code of Practice relates to all fertilisers sold under the Fertmark programme.

Farmers can be confident that fertilisers supplied from Fertmark registered companies meet the claimed nutrient content and, if used properly, do not pose hazards to food safety or animal welfare.

Fertmark provides quality assurance on claimed nutrient content to farmers purchasing fertiliser, with Fertmark certified products displaying a green tick.

To ensure ongoing product consistency, regular product samples are analysed, and procedures and results are subject to external, independent audit.

There are a number of similar products being sold in NZ of varying quality and price so it's best to check that you are actually getting what you are paying for and the Fertmark registered products are a good place to start. A prime example is Guano as there are a number of products out there that may have much lower phosphate levels in them than those that are Fertmark registered. The price may be a lot lower for non Fertmark registered products but in this instance you are most likely getting what you pay for... so beware.

The Guano that we sell is Fertmark registered and guaranteed to be a minimum of 10% phosphate.