Note that "play nicely" usage is increased by the fact that some usages of
PN do not suit 'nice' see at end.
And:
eg "I think they play nicely together" is euphonic to a NZ ear.
"I think they play nice together" is NOT euphonic to a NZ ear.

The 'nice' usage would almost always be used with a degree of
affectation/purposefulness in NZ despite its preponderance.
ie its a 'turn of speech'.

A look through about the first 50 "play nice" NZ gargoyle hits produced NO
instance where it was obvious that the user was illiterate or unaware that
"play nicely" was 'more correct. In ecery case Occam would suggest that the
term "play nice" was being used as a "canned phrase" and that the user was
quite liable to be aware that p.nicely was the more correct form if
 'noirmal grammar rules' were to be applied.

Web overall
nicely 2,140,000
nice
271000
7.9:1

NZ
Play Nice   5420
Play nicely 1300
~ 4:2

Oz
nice    32000
nicely   6990
~ 4.6

UK
nice 99100
nicely 38,700
~ 2.6

Canada
77700   Play nice
6510     Play nicely
~ 11.9

____________

Let's check NZ usage:

Actual posts that further skew the results towards "play nice"

 "Van Gaal is a coach who wants to *play nice* football and win games
that ..."

"I mostly *play nice* characters who do not cause troubles for others  ..."
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