Edited to add: This was sent to, not by, Henry Prinsep. There is no suggestion that Prinsep complied with the request.
Transcript follows. Many thanks to Vivienne.
COMMONWEALTH OF AUSTRALIA
POSTMASTER-GENERAL'S DEPARTMENT, WESTERN AUSTRALIA
20 JUL 07
TELEGRAM from Broome Station
Addressed to H. Princep Esq, prot. of aborigines
Send cask arsenic exterminate aborigines letter will follow