The colloquial Australian-English phrase to stir the possum, and its later variant to rouse the possum, mean:
– to stir up controversy;
– to liven things up.

According to the Australian National Dictionary Centre (Australian National University, Canberra, Australian Capital Territory):

[To stir the possum] probably developed as the obverse of the phrase to play possum meaning ‘to pretend to be asleep or unconscious when threatened’ (in imitation of an opossum’s supposed behaviour).

—Cf. meaning and origin of the phrase ‘to play possum’.

One H. V. Davies mentioned both to stir the possum and to play possum in the following letter to the Editor, published in The Sydney Morning Herald (Sydney, New South Wales, Australia) of Saturday 11^th November 1972 [No. 42,092, page 6, column 6]:

SIR—The term “stirring the possum” was old when I, with other youths in northern Victoria, spent many a night moonlighting possums by peering up through the branches of the gum trees endeavouring to catch the silhouette of the possum against the moon.
But the possum is a cunning little animal and will freeze against a limb if he sees an intruder, and he is very hard to detect.
A few stones thrown up among the branches “stirs the possum” and he becomes clearly visible against the moon.
All that was over 60 years ago and the terms “playing possum” and “stirring the possum” were common usage among the youth of my vintage.

The earliest occurrences of the phrase to stir the possum that I have found are as follows, in chronological order:

1-: From a correspondence from the town of Casino, New South Wales, dated Saturday 9^th June 1888, published in The Australian Town and Country Journal (Sydney, New South Wales, Australia) of Saturday 16^th June 1888 [Vol. 37, No. 962, page 1,211, column 4]:

The River Entrance.—Meetings have been held in all parts of the district in favor of the necessary works to the entrance of the river; Sir John Coode’s report having “stirred the ’possum” in a surprising manner.

2-: From a letter to the Editor, by a person signing themself ‘Snapper Catcher’, published in The Scone Advocate (Scone, New South Wales, Australia) of Saturday 24^th May 1890 [Vol. 2, No. 34, page 3, column 1]:

Sir,—In your last issue I seemed to have stirred the “’possum” in Mr. Horatius. I think he has made a great many errors in several things he said regarding the Lower Dartbrook. Mr. H. is still confident about the number of sheep in his district, but I think there are as many, if not more, sheep on the Lower Dartbrook as there is in Mr. H’s. district.

3-: From Melbourne Racing Notes, by ‘Martindale’, published in the Evening News (Sydney, New South Wales, Australia) of Tuesday 13^th October 1891 [Vol. 4, No. 7,605, page 6, column 2]:

Melbourne, Monday.—[…] To use a colonial phrase, the race for the Caulfield Stakes appears to have “stirred the ’possum” in Marvel. This morning, on the track, he was greatly excited, and gave Harris plenty of trouble.

4-: From Sporting Notes from Queensland, dated Brisbane, Sunday 5^th June 1892, published in The Referee (Sydney, New South Wales, Australia) of Wednesday 8^th June 1892 [No. 293, page 4, column 6]:

Clinker was again successful in the Juvenile Stakes on the second day, but this time he got a bit of a fright when Tridentate, a two-year-old, by Trident, and belonging to Mr. Herbert Hunter, came at him like a perfect bull dog, and he only just squeezed home by half a head. One stride more and the Trident colt would have had him good. This Tridentate is a chestnut with a white hind foot, and is said to be very much like the old horse. He is one of the right sort, an easy-going, lazy beggar, who takes things as they come, but when the lash stirs up the possum in him he shows what a game youngster he is, for he never flinches under punishment, and is true to the tip of his tail.

5-: From The Ballarat Star (Ballarat, Victoria, Australia) of Tuesday 23^rd August 1892 [Vol. 37, No. 200, page 2, column 8]:

In returning thanks to the electors of Dundas who had voted for him, Mr Spence claimed that though defeated he was not disgraced. He had had to fight under some disadvantages, but Mr Thomson had acted most honorably, as also had his committee. There had been no bribery of any sort. He had not spent a sixpence in influencing anybody, neither had the other side that he knew of. Though defeated he was not disheartened, and meant to go into Parliament at the first available opportunity, especially as there was such a desire on the Conservative side to keep him out. As a “backblocker” would say, they had “stirred the ’possum in him,” and if the present Government was put out and a dissolution followed, he would be found fighting for Dundas again. He did not intend to kill anybody to get a seat, but he meant to get one as soon as he could.

6-: From a correspondence from Parkesbourne, New South Wales, published in The Goulburn Herald (Goulburn, New South Wales) of Wednesday 3^rd May 1893 [page 2, column 6]:

The Farm.—Our farmers have been extra busy with the tillage of the land, but it is to be feared that the present wet weather will check their progress for a time, for rain has again set in with every appearance of continuing. I do not know what has revived the spirit of our farmers this season in this locality, unless it is the present high price of wheat and hay. Certainly something, to use a colonial phrase, has “stirred the ’possum” in them, for I notice that old paddocks that have been idle for years are this year being cultivated.

The earliest occurrence of the variant to rouse the possum that I have found is from a correspondence from Tilba Tilba, New South Wales, published in The Cobargo Chronicle (Cobargo, New South Wales, Australia) of Friday 9^th December 1898 [Vol. 1, No. 4, page 2, column 5]:

My letter of a fortnight ago, re the expenditure on the aborigines at the Wallega Lake settlement seems to have “roused the possum” in the editor of the “Times,” who launched out in a column or so of abuse, etc., on my luckless head.