This is the definition of the Australian-English phrase Fremantle doctor, from A Popular Dictionary of Australian Slang (Melbourne: Robertson & Mullens Ltd., 1941), by Sidney John Baker (1912-1976)—as quoted by Gerald Alfred Wilkes (1927-2020) in A Dictionary of Australian Colloquialisms (Sydney University Press in association with Oxford University Press Australia, 1990):

A refreshing sea-breeze that blows into Fremantle and Perth ¹ after hot weather, especially in the evening.

¹ Fremantle is a port city in Western Australia, near Perth, the state capital.

The noun doctor denotes, in Western Australia and in the West Indies, a cool sea-breeze which usually prevails during part of the day in summer.

This use of the noun doctor is often said to merely refer to the relief that a refreshing sea-breeze brings on hot days, but, in fact, the reference was originally to the action of an onshore breeze against diseases. This is clear in the below-quoted passage from The Herald (Fremantle, Western Australia) of Saturday 1^st February 1873; this is also clear in the following from A New History of Jamaica, from the Earliest Accounts, to the Taking of Porto Bello by Vice-Admiral Vernon. In Thirteen Letters from a Gentleman to his Friend (London: Printed for J. Hodges, 1740):

About Nine in the Forenoon ’tis intolerably hot, and could scarce be endured, did not the Sea-breeze, which generally begins to blow about that time, temper it, and make the Warmth so moderate, that Persons can apply to Business, and the Negroes work in the Fields; and, without Doubt, were it not for the kindly Effects of the constant Winds, which blow from Nine till Five, no Creature could inhabit here. […] The People here give it the Name of Doctor, and truly it deserves the Title; for did it not blow, How dismal would the Consequence be? The hot and moist Temperament of the Air would soon bring on Plagues, and other epidemical Distempers, and in a short time turn all to a Desart ²; but these Evils are provided against by the wise Contriver of Things, who has made these friendly Gales to blow, and temper the Air, that we need not be afraid of such Evils, or hindered going about our lawful Employments.

² In the 18^th century, desart was the regularly accepted spelling of the noun desert, denoting an uninhabited region.

The earliest occurrences of the phrase Fremantle doctor that I have found are from The Herald (Fremantle, Western Australia):

1-: Of Saturday 4^th January 1873:

The Weather.—Parlour fires have been at a discount during the last few days. People were able to refrain from luxuries of that kind. There may, perhaps, be one hotter place, but there is no one about who has tried both, and is in a position to give the result of his experience. There may have been warmer days than last Sabbath, but a hotter night never. The nearest approach to bed-clothes, we have heard, was the mosquito net at an elevation of six feet, and that was oppressive. Without paying much attentention [sic] to atmospheric phenomena we think that there is a noticeable change in the average run of weather. The old formula was simple. Three or four days of a fierce westerly wind, succeeded by a strong, cool sea breeze—known up the country as the Fremantle doctor—and, probably, a deluge of rain, the wind going round by the east. During the last year or two our changes have been from hot to hotter, until we arrive at the most seething heats of the tropics, without any lasting change of direction of wind. Perhaps the meteorologists who figure so conspicuously in the columns of our contemporary the Inquirer, could explain the reason of these altered weather characteristics.

2-: Of Saturday 1^st February 1873:

The Weather has been hot and most unhealthy. An epidemic cold and cough, most resembling a feverish influenza has traversed the country, and sickness has been in every household. We have heard of no cases in which this particular disease has terminated fatally; but our medical practitioners have been kept hard at work battling with an insidious foe, from whose attacks children appear to suffer most. The symptoms of the disease are intense langour [sic] and a racking cough which in many instances produces great nausea and vomiting. A change in the weather which took place the other day will probably do more than anything else towards dissipating the epidemic and removing its effects. For weeks the air has been sultry and smoke laden. Bush fires to a considerable extent have occurred in all parts of the colony, and the heat of the earth combined with the scorching glow of the sun created an atmosphere which it was well-nigh burdensome to live in. Now, however, a strong cool breeze, known up country as “the Fremantle Doctor,” is whistling with a will from the southward. We value highly medical skill; but we are truly delighted that this ally of the faculty has put in an appearance to help his brethren by clearing away the miasmatic influences which have given them so much to do.

The phrase also occurs as Freo Doctor and Freo Doc, after Freo, shortened form of Fremantle.

For example, the following is from The Australian (Sydney, New South Wales) of 2^nd June 1982—as quoted by Gerald Alfred Wilkes (1927-2020) in A Dictionary of Australian Colloquialisms (Sydney University Press in association with Oxford University Press Australia, 1990):

The Freo Doc is on the honker . . . It and other westerly zephyrs stink to high heaven—because they are laden with the effluvium of live sheep loaded on ships for export to the Middle East.

And the following is from The Age (Melbourne, Victoria) of Thursday 8^th January 1987:

There is no denying that the commercials, and even Auntie ³, are all putting up a mammoth effort to keep us up to date with the latest on the America’s Cup goings-on. What a pity then that a couple of basic things irk so much: like the mispronunciation of Fremantle (it should be Free-mantle) and the use of the term “Fremantle doctor” for anything that blows off the west coast. The “Freo Doctor” is the sea breeze which tends to arrive in late morning or afternoon in summer to cool things down, hence the name. The strong wind which caused Monday’s postponement of the Kookaburra II-Australia IV grudge race, might have been bad medicine but it was definitely not the doctor.

³ Auntie is a familiar name for the Australian Broadcasting Corporation.

The Macquarie Dictionary defines a similar phrase, Albany doctor, as:

A cooling afternoon sea breeze which arrives in Kalgoorlie ⁴.

⁴ Albany is the southernmost port city of Western Australia; Kalgoorlie is a city in the Goldfields–Esperance region, in the south-eastern corner of Western Australia.

The earliest occurrence of the phrase Albany doctor that I have found is from From Albany to Katanning ⁵. No. II, published in The Kadina and Wallaroo Times (Kadina, South Australia) of Saturday 1^st June 1901:

Most of the rain falls from the north-west. The prevailing wind is from the south-east coming direct off the Southern Ocean, and is known as the “Albany Doctor.” The evenings are cool and a light dew falls towards morning, but it seldom or never freezes. There are no hot winds.

⁵ Katanning, in south-western Western Australia, is located approximately 95 miles north of Albany and 155 miles south-east of Perth.