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The colloquial Australian-English phrase couldn’t train a choko vine over a country dunny, and its variants, are used of an ineffectual person.

This phrase occurred, for example, in an interview of the Australian politician Scott Morrison (born 1968) by the Australian broadcaster Ray Hadley (born 1954), broadcast on Monday 18^th January 2016 on 2GB, a radio station in Sydney, New South Wales—in the following passage, Ray Hadley talks about the Australian businessman and politician Clive Palmer (born 1954):

This is the bloke who said […] that he would make a great Prime Minister. Honestly, a choko vine over an outhouse would be in strife with this bloke in charge.

The Australian and New Zealand noun choko designates the cucumber-like fruit of a cucurbitaceous vine, Sechium edule.—Synonym: chayote.

—Cf. also:
– the noun ‘dunny’ in Australian phrases;
– ‘may your chooks turn into emus and kick your dunny down’;
– ‘to bang like a dunny door’: meaning and early occurrences.

The earliest occurrences that I have found of the phrase couldn’t train a choko vine over a country dunny and variants are as follows, in chronological order:

1-: From The racing world of Bert Lillye, published in The Sydney Morning Herald (Sydney, New South Wales, Australia) of Thursday 23^rd October 1969 [page 13, column 3]:

YOU HAVE heard of men who could not train a pig to be dirty, or a choko vine over the back fence.
I have found the man who could train an elephant to do the Blondin act ¹ over Niagara Falls.

¹ Charles Blondin (Jean-François Gravelet – 1824-1897) was a French acrobat and tightrope walker, best known for walking a tightrope across Niagara Falls in 1859.

2-: From a letter by one N. Clarke, of Lakemba, New South Wales, published in The Bulletin (Sydney, New South Wales, Australia) of Saturday 31^st January 1976 [Vol. 98, No. 4,993, page 5, column 3]:

It is quite obvious that David McNicoll (B, January 10) believes that political campaigns are won from the top. […] He should have given his congratulations to the countless numbers of workers who generously gave their time and energy to the Liberal Party campaigns conducted in the marginal seats, which always decide the political persuasion of the incoming government.
During the campaign I remember one highly placed party worker saying to me that Ash Street ² couldn’t organise the growing of a chokoe vine over a country outhouse. We all know that it doesn’t require much organisation to grow a chokoe vine.

² The headquarters of the Liberal Party of Australia are in Ash Street, Sydney, New South Wales.

3-: From David McNicoll’s column, published in The Bulletin (Sydney, New South Wales, Australia) of Tuesday 23^rd October 1979 [Vol. 100, No. 5,182, page 55, column 3]:

READERS of this page will know that I have agonised for a long time about the attire of Phillip Adams ³, whose comfortable embonpoint is seldom lined externally by anything dressier than jeans and a black skivvy.
Judge, as they say, of my surprise when an apparition appeared in our office last week, the face unmistakably Phillip Adams, but the finery recalling Beau Brummell, Bertie Wooster and other assorted moulds of fashion.
It was Phillip. […]
The change has been as dramatic as it is pleasing. Will it change the Adams prose style? Will we no longer read of Jaffas rolling down the back stalls, and choko vines over the privy? Will we now hear only of muffins in the inner sanctum of the House of Lords, and smoked salmon sandwiches in Whites?

³ Phillip Adams (born 1939) is an Australian journalist, broadcaster and author.

4-: From The Sunday Carlton Report, by Mike Carlton, published in The Sun-Herald (Sydney, New South Wales, Australia) of Sunday 27^th December 1981 [page 99, column 3]:

Common as muck, you lot are. Off like a bag of prawns in the sun. You’d make a blowfly sick.
These are just some of the colourful Australian sayings that you so graciously submitted for the distinguished diplomatist Sir Leslie Patterson to use in his forthcoming London appearances.
[…]
Mrs Joy Sutton of Harrington, with “he couldn’t train a choko vine over a country dunny.”
Mrs A. Ogilvie of Baulkham Hills: “A mug is a mug, boiled fried or fricasseed.”
And H. Wright of Nelson Bay: “He couldn’t track an elephant with tar on its feet down Pitt St ⁴ in a heavy fall of snow.”

⁴ Pitt Street is a major street in the central business district of Sydney, New South Wales.