[Jimbuna]

In the most general terms:

Supply boats were already stocked with supplies prior to 'internment'.

Replenishment was done mainly during periods of darkness.

'Internment' was often a position of convenience to the host nation to appease the allies.

The allies were often aware and spies would report a U-boats presence, often affording the allies the opportunity to await the U-boat leaving said port in international waters.

Not all supply boats were in harbours, but often in open seas.