Lambing down has nothing to do with sheep or lambs.
Lambing down was the practice of taking your entire pay (cheque) for the season and giving it to the the barmaid (or publican). They would provide enough drink to keep the man drunk until he was told his cheque had run out.
The practice was against the law but magistrates would seldom convict the publican.
These drinking orgies resulted all too often in insanity and death, not just because of the quantity of alcohol they consumed, but also because they drank nobblers.
A nobbler is a small shot glass filled to the top with hard liquor laced with some other booze, drug or even poison that would give the drink a real hit.
One gallon of good rum could be doctored to make eight gallons of nobbled rum. Nobbled rum cost less to buy and got the person drunker faster and cheaper.
Depending on what the rum was mixed with, it could kill him when lambing down.
I'm a broken-hearted shearer, I'm ashamed to show my face,
I thought I was no flat, so resolved to cut it fat;
I would get up in the morning to have a glass of stout;
My money getting short I resolved to know my fate;
I had two-and-six in silver and half-a-bar of soap,
I had two old shirts, but they were all in rags;
Sung to the tune Wearing of the Green