If you do that by buying $10k x 10k transactions, your demand will increase the price of the asset and mean you pay more than current price for no good reason.
OTC desks either collect the $100m from large holders in big blocks, or they use working capital to constantly buy the asset and sell it for a single quoted price (hoping to gain on arbitrage).
In this particular case, they also offered the unique service of accepting /nzd vs. EUR/USD/JPY/KRW