The New-Zealand phrase (as) cunning as a Māori dog means very cunning.

This phrase occurs, for example, in Hush now, don’t cry ([Auckland]: AM Publishing New Zealand, 2010), a novel by Bob Ellis (born 1937):

“He’s as cunning as a Maori dog. Before ya know it he’ll have you ironing his clothes an’ cleaning his shoes. Smooth as shit he is.”

The phrase (as) cunning as a Māori dog refers to the noun Māori dog, which designates:
– a dog of Polynesian origin which was first introduced to New Zealand by the Māori and became extinct as a pure breed during the 19^th century, after the arrival of the European settlers.
– any mongrel dog associated with Māori settlements or living in a wild state.

The phrase (as) cunning as a Māori dog is not a loan translation from Māori. The following explanations are from Say it in Maori: Maori Phrase Book (Paraparaumu: Seven Seas Publishing Pty Ltd., 1968), compiled by Alan Armstrong:

AS CUNNING AS A MAORI DOG
Me te kiore haumiri kakaka.
The early settlers were impressed with the cunning of the dogs in the Maori villages and coined the above English expression. The Maori however took his simile from the rat creeping along the kakaka or lattice fence which protected the kumara plantations.

And, according to Manawa Toa: Heart Warrior (Melbourne: Spinifex Press Pty Ltd., 2000), by Cathie Dunsford, the Māori phrase me te kiore haumiri kakaka means, literally, like a rat hugging the fence.

The earliest occurrences of the phrase (as) cunning as a Māori dog that I have found are as follows, in chronological order:

1-: From The Waikato Independent (Cambridge, Waikato, New Zealand) of Tuesday 25^th February 1908:

A correspondent brings under our notice the wonderful sagacity of a native dog at Maungatautari. Some grass had caught fire and the canine, in order to attract the attention of some workmen, commenced to bark loudly. No particular notice was taken of the animal until a while after, when it was observed to plunge into a creek and afterwards shake itself near the fire. It repeated this performance several times, and there is no doubt, we are assured, that by so doing it prevented the spread of the fire. Yes, we have heard the phrase, “As cunning as a Maori dog,” but—pass the salt!

2-: From the column Billiards, by ‘Jigger’, published in the New Zealand Truth (Wellington, New Zealand) of Saturday 13^th January 1912:

John Roberts is reported to be going on with his action against George Gray for breach of contract. It is said that he holds a winning chance, but then George Gray is only a minor, and his old man is as cunning as a Maori dog, and pretty hard to settle.

3-: From the transcript (published in 1916 by the New-Zealand Government Printer) of a debate on the Native Land Amendment and Native Land Claims Adjustment Bill, which took place in the New-Zealand House of Representatives on Thursday 3^rd August 1916—William Thomas Jennings (1854-1923), Member of Parliament for Taumarunui, said the following:

I wish to refer, in passing, to the remarks about the Maori dog made by the Hon. the Minister in Charge of the Maori Councils and the member for Otaki. The Maori dog is a great nuisance in many places owing to the great numbers kept by our Maori friends; and no doubt at times these dogs are the cause of a great deal of destruction amongst the sheep not only of the pakeha farmer, but of the Maori farmer. It is no argument for the Minister to say that the European also has dogs. Let me remind him that there is an old saying that there is nothing so cunning as a Maori dog. It is very difficult indeed to find out where he gets to at times, when he is hungry and wants a bit of mutton.

4-: From The New Zealand Herald (Auckland, New Zealand) of Saturday 10^th October 1931:

WANTON SPEECH.
BY POLONIUS
OF ANIMALS AND OTHERS.

Poor, abused, maligned animals! Whenever human beings desire to humiliate an object of their dislike or to describe a particularly unpleasant situation, almost invariably they liken that object or situation to some unfortunate member of the animal kingdom. The tragedy of the thing is that the English language (in common with others) does not appear to contain sufficient words adequately to express strong feeling without this obnoxious practice.
[…]
[…] If a person consume [sic] his food ravenously for any reason, he “hogs” or “wolfs” his food. Should he take more than his share, he is “as greedy as a pig.” Does the pig know the share or portion allotted to him? If the poor brute could reason, he would know that the more he ate the quicker he would be killed.
“As sly as a fox” and “as cunning as a Maori dog” are not so indefensible, because there is more than a little truth in them. “As hungry as a wolf” may pass. “A hang-dog look” is probably a fairly mournful one but—que voulez-vous?

5-: From the column News of the Day, published in The Auckland Star (Auckland, New Zealand) of Wednesday 20^th March 1935:

A Maori Dog.
“As cunning as a Maori dog” is a term that has been frequently used. The survey party from Wanganui engaged on Duthie’s track at Matahiwi for some weeks had several experiences of the cunningness of a Maori dog, and their larder was depleted on more than one occasion. “In fact,” remarked one of the party, “the dog was so shrewd that if we nailed down the box with our provisions the dog would almost draw the nails. After the first raid we put a heavy camp oven over the box, but the dog managed to lever this off with its nose. We also had the experience of it making a very good bid to open our meat safe.” The same dog also paid more than one visit to the camp of a well-known Wanganui bridge contractor, who is erecting the new bridge over the Matahiwi Stream. The canine always operated at night time, and when it was identified and pointed out to a Maori he replied: “Py korry, that not te dog; him always chained up at night time. Him also have te distemper, and not eat very well for te long time now.”

A synonymous New-Zealand phrase is (as) cunning as a Māori hen, which refers to Māori hen, designating a weka, or woodhen, i.e., a flightless bird of the genus Gallirallus, endemic to New Zealand.

The earliest occurrence of this phrase that I have found is from In Sanctuary, by ‘Tangiwai’, published in The Auckland Star (Auckland, New Zealand) of Monday 20^th May 1935:

There is the grey duck, more fortunate than the pheasant family in the matter of safety areas. With its decreasing numbers, it has perforce developed an acute cover-taking sense. Some of our gunning sports come home loaded with pride and dead birds; others complain bitterly that the duck is becoming as cunning as a Maori hen and takes off for the sanctuary lagoons at the first sound of a gun. It is disgusting to the sporting mind to find the open lakes bare of fowl life and the adjacent sanctuary waters covered thick with chuckling duck. The more intelligent birds, the flock or covey leaders, are popularly believed to consult calendars and tick off the days until the first of May, by which time they have consumed most of the food in the open waters, and are ready to take flight for the peace and plenty of such sanctuary waters as their friends of the Native Birds Protection Society have induced the Government to reserve for them. A curious thing, this self-defence instinct created, or rather sharpened, by the intensive May campaign of sport-loving man.