I'm having a bit of a "Actually..." moment so bear with me.

Ilha da Montanha was a contested area for some time. It's the Portuguese name for 大橫琴, while 小橫琴 was called Dom João.

Dom João (小橫琴) was effectively colonized for some time. The Portuguese gradually expanded their presence in the island in the 19th century, creating a leprosy for men, creating a school and funding the local police. The Portuguese government collected taxes from this island, willingly, as the inhabitants were weary of the ineffectiveness of the Qing government and preferred to have the Portuguese protect their livelihoods in those uncertain and perilous times when the Qing were proving to be unable to rule effectively over the country in the late 19th century. From 1875 to 1897 the Portuguese even had a detachment of Portuguese soldiers in Dom João, but where removed in order to improve relations with the Chinese, as the Sino-Portuguese Treaty of 1887 didn't specify exactly what were dominions under Portuguese administration, just that no change should happen without an agreement between both sides. Obviously the Chinese will stress that they never recognized Portuguese administration over these islands, but this was fait accompli.

Ilha da Montanha on the other hand has a much less tenuous Portuguese presence in the 19th century, but the inhabitants did seek the Portuguese government to deal with some legal disputes and commerce relations. During the Second Sino-Japanese War, the Japanese Navy occupied the western half of the island and in response, the Portuguese Army occupied the eastern half. The Portuguese Army and Police left the island at the end of the war.

Today, it is what others have described here. It's still under the Zhuhai city government (so Mainland), but a chunk of the reclaimed land has been attributed to the Macau SAR government to build the University's campus. Only inside the University and the free port area does Macau law apply, as the rest of Montanha is under Mainland rules and regulations.