Terra Nullis

well, the British were taking over MOST of their empire at the time......they still had Batavia (aka Java aka Indonesia)

They lost most of their territory at the Congress of Vienna in exchange for Belgium, which they didn't even keep for very long. The major territory taken by the British was new Amsterdam, in exchange for Suriname, which was the biggest fleece ever until deandre Hopkins was traded to the Arizona Cardinals.

yeah well the Congress of Vienna was early 19th C, they kinda were losing territory to the British well before that...the Second and Third Anglo Dutch Wars kinda nixed their (the Dutch) ability to colonize Australia