The Australian Customs Flag is the flag flown by Australian Customs Service

Australian Customs Service

The Australian Customs and Border Protection Service is the Australian Federal Government agency responsible for managing the security and integrity of the Australian border, facilitating the movement of legitimate international travellers and goods, and collecting border-related duties and...

 vessels and sometimes on ACS buildings. Any vessel acting in a customs capacity must fly this flag. The current version is an Australian National Flag

Flag of Australia

The flag of Australia is a defaced Blue Ensign: a blue field with the Union Flag in the canton , and a large white seven-pointed star known as the Commonwealth Star in the lower hoist quarter...

 with the word "CUSTOMS" added in bold between the Commonwealth Star and the lower part of the Southern Cross

Crux

Crux is the smallest of the 88 modern constellations, but is one of the most distinctive. Its name is Latin for cross, and it is dominated by a cross-shaped asterism that is commonly known as the Southern Cross.-Visibility:...

. This flag has been used since 1988.

Colonial customs flags

English law has required customs vessels to fly a distinctive flag since at least 1784. The earliest recorded Customs flag in Australia is the New South Wales

New South Wales

New South Wales is a state of :Australia, located in the east of the country. It is bordered by Queensland, Victoria and South Australia to the north, south and west respectively. To the east, the state is bordered by the Tasman Sea, which forms part of the Pacific Ocean. New South Wales...

 Customs House Ensign of 1832, which included in the Code of Signals for the Colony of New South Wales in the NSW Calendar and Post Office Directory for that year. The flag was a British Red Ensign

Red Ensign

The Red Ensign or "Red Duster" is a flag that originated in the early 17th century as a British ensign flown by the Royal Navy and later specifically by British merchantmen. The precise date of its first appearance is not known, but surviving receipts indicate that the Navy was paying to have such...

, defaced

Defacement (flag)

Defacement is a term used in heraldry and vexillology to refer to the addition of a symbol or charge to another flag. For example, the Australian flag is the British Blue Ensign defaced with the Southern Cross in the fly and the Commonwealth Star in the lower hoist quarter, beneath the Union...

 with a gold crown over the letters CH in the fly.

A later New South Wales Customs Colonial Flag was described by a regulation published in the Supplement to the NSW Government Gazette, No. 193, Friday, 12 May 1882:

"The proper ensign for Customs shall be the red English
ensign with the addition of a white cross, being in the
form and proportion the same as the white ensign, but
with the colours of the flag reversed, and with the letters
CH in the outer lower quarter of the flag; and the pendant
shall be the red pendant."

Commonwealth customs flags

The Customs Act 1901 was passed soon after federation

Federation of Australia

The Federation of Australia was the process by which the six separate British self-governing colonies of New South Wales, Queensland, South Australia, Tasmania, Victoria and Western Australia formed one nation...

, and like previous British and colonial legislation, required the use of a customs flag. The first flag appointed under this act was specified in Section 14 of the Customs Regulations, which were gazetted on 1901-10-01 in the Commonwealth of Australia Gazette No. 53, page 172:

"The Customs flag shall be the Blue Ensign

Blue Ensign

The Blue Ensign is a flag, one of several British ensigns, used by certain organisations or territories associated with the United Kingdom. It is used either plain, or defaced with a badge or other emblem....

,
with the addition in the fly of the letters "HMC" in bold
character, and the word 'Australia'".

An order in Council, dated 1904-06-16, altered the regulations to remove the word "AUSTRALIA" from the flag.

The change to the current version of the flag was achieved by amendment SR297, published 1987-12-17 and commencing 1988-01-01.