The colloquial Australian and New-Zealand phrase green cart designates the imaginary vehicle supposed to take people to the mental asylum.

This phrase occurs, for example, in the following from Lily on the Dustbin: Slang of Australian Women and Families (Penguin Books Australia Ltd., 1982), by the Australian author Nancy Keesing (1923-1993) [page 164]:

Green cart: vehicle allegedly sent to convey mad people to the asylum. ‘He wants to look out, they’ll be sending the green cart for him next.’ Pronounced almost as one word, greencart. (This is akin to a threat of my city childhood, ‘the black Maria’ will get you. In the 1920s and 30s police lock-up vans were cumbersome black vehicles.

The reason the colour green was chosen to characterise the vehicle is unknown. The texts containing the earliest occurrences that I have found of green cart provide no clue in that regard. Additionally, the authors of those texts did not feel the need to explain the meaning of the phrase, which indicates that it was already in common usage—those early occurrences are as follows, in chronological order:

1-: From a letter to the Editor, dated Bourke, Monday 10^th May 1869, by a correspondent signing themself ‘Spectator’, denouncing sly grog (i.e., liquor sold without a licence), published in the Pastoral Times (Deniliquin, New South Wales, Australia) of Saturday 22^nd May 1869 [Vol. 12, No. 537, page 3, column 2]:

I certainly was not prepared to see in print such unmitigated trash as that which appeared in your issue of the 1^st instant from your Paroo correspondent; he states in such a barefaced way that the violation of the law is a commendable thing, and the production is altogether such an incongruous mass, that I think he must have over-indulged in the good cheer provided by the friends whose cause he espouses—otherwise I should strongly recommend the green cart to take a turn up the Paroo, as he surely must have got a touch of the Darling pea [note 1], and he might become dangerous.

2-: From a correspondence from Upper Darling River, dated Monday 27^th May 1872, published in the Pastoral Times (Deniliquin, New South Wales, Australia) of Saturday 8^th June 1872 [Vol. 15, No. 696, page 3, column 1]:

The copper mines about Wilcannia are commencing to look up. I hear that in the “Great Wilcannia” they have struck a very rich lode, and shares have advanced in consequence. I happened to be in Wilcannia a few weeks ago, and stopped one night—if I had stopped there any longer I would have had “copper on the brain.” I would suggest that the “green cart” be sent there at once.

3-: From a correspondence from Longford, Tasmania, dated Wednesday 23^rd September 1874, published in the Launceston Examiner: Commercial and Agricultural Advertiser (Launceston, Tasmania, Australia) of Thursday 24^th September 1874 [Vol. 34, No. 115, page 3, column 6]:

From what I hear there will be very little for the “green cart” folk to do here this year—in re railway rate—out of about £2000 due there is only something like £300 uncollected.

4-: From a correspondence from Emu Bay, Tasmania, dated Saturday 17^th July 1875, published in the Launceston Examiner: Commercial and Agricultural Advertiser (Launceston, Tasmania, Australia) of Thursday 22^nd July 1875 [Vol. 35, No. 87, page 3, column 2]:

How is it that the Cornwall Chronicle arrives here so punctually, while the subscribers to the Examiner have to wait for the steamer? I have made particular enquiry here, and find that every subscriber to the Chronicle gets his paper by mail, while the Examiner men go to bed news hungry. This is not fair and should be seen to. What a pity the mail contractor was not one of those wiseacres that “Who” attempts to describe who do better on a bad road than a good one. Some further information respecting these human curiosities might lead if not to public benefit, at least to the visit of the green cart to somebody’s neighborhood.

5-: From the column Victorian Notes, a correspondence from the state of Victoria, published in The Border Watch (Mount Gambier, South Australia, Australia) of Wednesday 8^th December 1875 [Vol. 15, No. 1,262, page 4, column 1]:

The Victorian people seem to have railway on the brain at present, and in the event of a general election each new candidate will be suggesting a new line with the green cart at the end of it.

The earliest New-Zealand use of the phrase green cart that I have found is from a correspondence from Wairio, in Southland, published in The Southland Times (Invercargill, Southland, New Zealand) of Friday 15^th February 1884 [No. 4,864, page 4, column 1]:

I notice of late a gigantic scheme for the eradication of the rabbit pest has been propounded, viz., the destruction of all harbor for rabbits. Anyone advocating such a scheme must be surprisingly ignorant of the difficulties to be encountered in some localities. To associate the idea with any of the rugged country which forms a great portion of New Zealand, one would require to draw extensively upon the imagination, and to attempt to practice it would simply merit a trip in the “green cart.”

Note 1: The noun Darling is used attributively in the names of certain plants growing in the neighbourhood of the Darling River, in New South Wales. The following explanations are from The Australian Language (Sydney and London: Angus and Robertson Ltd., 1945), by Sidney John Baker (1912-1976) [chapter 3: The Soil – page 63]:

Outback workers are subject to a number of maladies which have acquired localized names. Among them are: […] Darling Pea, upon which Stephens and O’Brien [note 2] offer this comment: “One of Australia’s poisonous plants; cattle eating it become afflicted with staggers and die: so a man wandering in gait or dazed in appearance is said to be suffering from Darling Pea.”

Note 2: This refers to Material for a Dictionary of Australian Slang 1900-1910, an unpublished manuscript by S. E. O’Brien and A. G. Stephens.