Police in Australia have issued a nationwide alert following the discovery at a sorting office in Canberra of at least twenty letter bombs addressed to local tax collectors.

They were discovered after one exploded, slightly injuring two postal workers.

An Australian Tax Office official said it was not known who planted the bombs or why, but he said all members of staff and their families, had been put on alert.

A Canberra police spokesman said the letter bombs were sophisticated and well-constructed.