!00AUD= 93.10NZD

It is the question that is inaccurate.  GoldenLeaf's working appears to be good.      

https://www.google.com.au/webhp?sourceid=chrome-instant&ion=1&espv=2&ie=UTF-8#q=conversion%20australian%20dollars%20to%20nz%20dollars

Use a ration to solve.

So if 93.1 NZD is equivalent to 100 AUD...

$${\frac{{\mathtt{93.1}}}{{\mathtt{100}}}}$$

We can use 788 NZD as a numerator to find how much it would be in AUD.

$${\frac{{\mathtt{93.1}}}{{\mathtt{100}}}} = {\frac{{\mathtt{788}}}{{\mathtt{x}}}} \Rightarrow {\mathtt{x}} = {\frac{{\mathtt{788\,000}}}{{\mathtt{931}}}} \Rightarrow {\mathtt{x}} = {\mathtt{846.401\: \!718\: \!582\: \!169\: \!71}}$$

The value of 788 NZD in AUD is ~846.40 AUD

How did you come with more AUD when converting from less NZD to AUD?? Australian Dollar has never been weaker than New Zealand's dollar!! It is not a matter of simple maths calculation with random rates.