Green’s Dictionary of Slang

1. a square gin bottle, often used to hold beer.

1909		Truth (Wellington) 3 July 5: Square-riggers of beer and small flasks of brandy were the favorite [p]otions the ladies lovingly lapped [DNZE].
1910		Truth (Wellington) 4 June 7: It is not quite certain, at least in the police court, what one should call the bottles [...] A good old-fashioned name is a ‘square-face,’ but in the court the other day ‘square rigger’ was the term used [...] while Counsel went so far as to call it a ‘square-rigged bottle.’ [DNZE].
1931		F. Cloke Songs of N.Z. 130: Rise, ye bushmen and gumdiggers, / [...] Where you’ve sunk with your square-riggers, / With your brain on fire with booze!
1941		NZEF Times 4 Aug. 4: What we really need is a few more square riggers [DNZE].
1962		P.A. Lawlor More Wellington Days 21: A ‘square rigger’ (a full-sized empty gin bottle) would be filled for 8d.
1981		C. Hunt Speaking a Silence 126: Old Gus eventually trotted out with his square rigger to take on his way. (Those riggers were the old, square gin bottles, too. They held easy a quart of grog.) [DNZE].
1988		McGill Dict. of Kiwi Sl. 106/2: squarerigger half-gallon jar of beer, named after gin bottles recycled for beer pre-WWII.
2003		McGill Reed Dict. of N.Z. Sl. [as cit. 1988].

2. a half-gallon (3 litre) flagon of beer; thus the beer itself.

1908		N.Z. Truth 8 Feb. 6/3: The excuse given by Taylor [...] was that he was full of square rigger and that his brain was paralysed.
1910		N.Z. Truth 22 jan. 7/1: When [Constable[ Roach entered he found a ‘square rigger’ of beer.
1919		Camperdown Chron. (Vic.) 6 Sept. 1/6: I suggest that the accused uses the hand-bag to carry two square riggers of beer.
1922		N.Z. Truth 15 Apr. 6/3: The indefatigable officer discovered a ‘square rigger’ of draught beer under the mattress.
1988		McGill Dict. of Kiwi Sl. 106/2: squarerigger half-gallon jar of beer, named after gin bottles recycled for beer pre-WW2; Squarerigger Gully a working class inner suburb of Wellington so called because of men coming home from the pub up Aro Street with their squareriggers of beer.
2003		McGill Reed Dict. of N.Z. Sl. [as cit. 1988].