I have three covers that have been posted in the Milne Bay Buna area at the start of WW2.

Two covers just have a three (3) bar cancellation with a violet circle and a dot in them.

One has no stamps available but the violet circle with the dot.

The circle is about the size of a the end of a pencil with the dot in the centre.

Can anyone help with the place reason and timings of the use of this style of cancellation.