The environmental group, Greenpeace, says the two freighters carrying nuclear fuel through the Pacific have been moving through its protest line during the night.
The protest flotilla of ten boats had positioned itself between Lord Howe and Norfok Islands.
The Nuclear Free Seas Flotilla launched an inflatible craft to follow the ships.
Two swimmers were then dropped into the water and they held up a "Nuclear Free Pacific" banner.
The British Nuclear Fuel transport ships are returning a cargo of reprocessed mixed plutonium and uranium oxides to Britain, after Japan rejected the fuel because of falsified safety data.
A spokesman for the Flotilla, Henk Haazen, said the decision by the freighters to move through the vessles at night was an indication that they were humiliated by the presence of the anti nuclear protestors.
At the end of last week, a summit of African, Carribean and Pacific countries being held in Fiji included a condemnation of nuclear shipments in a formal declaration.
They called for an immediate end to such shipments inorder to prevent any accidents that could seriously threaten a country's sustainable development and the health of their peoples.