This is the definition of the Australian-English phrase ducks on the pond by the Australian National Dictionary Centre:

Look out – female approaching! A warning cry from a male as a signal to other men that a woman is approaching a traditionally all-male environment. It is a reminder that the men should modify their language and behaviour to avoid giving offence. It was first used in shearing sheds, but is now heard in other places, especially in a pub. While the first written evidence comes from the early 1980s the phrase probably goes back several decades earlier.

In an interview published in Tharunka ¹ (Kensington, New South Wales) of Tuesday 4^th April 2000, Anne Summers (born 1945), a prominent Australian feminist, confirmed that the phrase was in use before the 1980s—in the following passage, she was talking about one of her books, Ducks on the Pond: An Autobiography 1945-1976 (Ringwood, Victoria: Viking, 1999):

The title comes from a phrase which I first heard in the late Sixties, and which is apparently still used by shearers today to signal that a woman is approaching the shearing shed. It was a warning to stop swearing, or even stop work, until she had gone. It struck me as an apt metaphor for the place women used to hold outside the home in Australian life, in the decades before the Women’s Liberation movement: women were seen as intruders into men’s territory.

¹ Tharunka: the Journal of the University of New South Wales Students’ Union.

These are, in chronological order, the earliest occurrences of the Australian-English phrase ducks on the pond that I have found:

1-: From the chapter ‘Ducks on the Pond!’ of The Shearers (Melbourne: Thomas Nelson Australia, 1982), by the Australian author Patsy Adam-Smith (1924-2001):

Usually the wool presser calls first, because most visitors to a shed come through the wool room. ‘Ducks on the Pond!’ he calls, and the warning is taken up again down along the shearing board.

2-: From Woman they couldn’t stop, by Jenny Stevens, published in The Sun-Herald (Sydney, New South Wales) of Sunday 14^th December 1986:

Miss Gaudron ² came up against prejudice of all forms in her legal work. Once when she was hearing a shearing case, the shearers refused to let her enter the sheds.
Whenever she approached, the men would down shears, yelling “ducks on the pond”. Eventually, she got into a shed, set up a hearing, and was delighted to notice she was sitting on a bale of wool, announcing she always had an ambition to sit on the woolsack ³, the traditional seat of Britain’s Lord Chancellor.

² Mary Gaudron (born 1943) was then the Solicitor-General of New South Wales.
³ This refers to the noun woolsack in the sense of the usual seat of the Lord Chancellor in the House of Lords, made of a large square bag of wool without back or arms and covered with cloth.

3-: From Glancing Blows: Life and Language in Australia (Penguin Books Australia, 1987), by the Australian author Alexander Buzo (1944-2006):

A shearer’s wife told me that when a woman entered a shearing shed the first man to see her called out, ‘Ducks on the pond, boys’. This was a coded signal to stop swearing and smarten up.

4-: From Where shearers’ legends live on, by Peter Cole-Adams, published in The Age (Melbourne, Victoria) of Wednesday 16^th December 1987—Charlie Davidson has been a shearer for most of his life:

[Charlie] recalls the days, not so long ago, when the arrival of women visitors at a shearing shed was greeted by the muttered warning, “Ducks on the Pond”, so that the men would know to moderate their language.