During his time heading the police force, Dunman was known for being on good terms with the people of various classes and communities within Singapore, and thus able to gain assistance and first hand information regarding what was happening in the city.

During this time, Singapore was flooded with new immigrants who often got to the then British Crown Colony though illegal means from non-British controlled part of Malaysia such as Kuala Lumpur, and also snuck in from hidden cabins in ships from India and Southeast Asia, often hiding illegal and contraband items such as drugs especially opium for sale in Singapore then with the colonial British government profiting off colonial slaves called "coolies" making them work for free by addicting them and selling them high-priced opium in opium dens.

As a result, many Chinese girls died of sex diseases, some were drowned during their voyage to Singapore, still many others committed suicide or were murdered by criminals in secret societies, when they could not repay their "slave debt" and the high interest piled on them when the colonial banking industry headed south.

At one point, almost all the women in Singapore died out, and prostitutes had to be imported from Macau, then a Dutch province, to supply more sex slaves to the colony so the British port could continue to collect high rents and sell land to shady criminals who hoarded tenancies and land for vices in the sex trade, the slave trade and illegal "ventures" in gambling and racketeering.

Dunman retired from the police force in 1871, and spent the next few years on his coconut plantation, Grove Estate (in what is now the Mountbatten area of Katong).