The Australian-English phrase like a stunned mullet means dazed and uncomprehending.

There are several ways of stunning mullet. C. Thackeray evoked one in Fishing. Trip to Wattemolla, published in The Sun (Sydney, New South Wales) of Sunday 7^th May 1916:

Eight miles from Audley, Port Hacking, the fisherman reaches Wattemolla, a pocket inlet of rare beauty, a little to the south of Jibbon Reef. Wattemolla is mentioned in the Tourist Department publications as a tourist resort where fishing is obtainable. […]
[…]
Dynamiting has been carried on at Como and Port Hacking, stunned mullet and tailer being picked up by boat parties after the Sinn Feiners have been operating at night. Round Ship Rock (Port Hacking), where bream had been plentiful, some explosives were used last Saturday night, and next day it was almost impossible to raise a fin there on a line.

‘Wobbegong’ described another way of stunning mullet in the column Fishing, published in The Sun (Sydney, New South Wales) of Sunday 16^th March 1919:

A novel way of catching mullet was mentioned at an A.F.A. meeting recently. Mr. J. C. Royle said that on the Clarence, when he was a boy in the seventies (or was it the sixties), he and other lads used to throw stones among the shoals of mullet, and the shock used to stun the fish, which floated, and were picked up suffering from concussion. Mr. T. Richardson went a bit further. He said he saw boys throwing stones into a waterhole, formerly a brick pit, at Marrickville, and stunning large mullet, which they secured. He wondered how the mullet got into the waterhole. Another member said he had seen a man throwing a tomahawk into the upper part of the south-west arm at Port Hacking and stunning mullet in shallow water. It seems as if the operations that “our black brudder” practised have been handed down to us.

These are the earliest occurrences of the phrase like a stunned mullet that I have found, in chronological order:

1-: From The Examiner (Launceston, Tasmania) of Friday 11^th January 1918:

Vivid word pictures of the attacks by Australian troops in front of Ypres and at Passchendaele are given by an infantry officer in the course of a letter to friends in Melbourne. The officer writes under date of October 27:—
[…]
“We made our first attack on October 4, when we took Zonnebeke, the station, and two commanding ridges and consolidated our position two miles into Boche territory. We had New Zealand troops on our left, and another Australian brigade on our right, so we felt safe, and were confident from the start. After having a bad time on our approach march—when he shelled us all along our track, we finally dug into shell holes in the dark opposite the Boche trenches, and waited there like ‘stunned mullets’ for three hours with the Huns shelling us. It was rather tense waiting for 6 a.m., the commencement of our big attack on the 12-mile front.”

2-: From the column Gossip of the Turf, by J. M. Rohan, published in The Sporting Globe (Melbourne, Victoria) of Wednesday 9^th October 1940:

All the week end I have been walking round like a stunned mullet wondering why I didn’t convert the family jewels into cash and have a headlong tilt at Ajax in the Melbourne Stakes for for [sic] which the bookmakers were prepared to take 2/1. Out of every 1000 people who visited Flemington, 999 thought Ajax a 10-1 on shot. The champion opened at 6-1 on, yet in a few minutes it was possible to lay 2-1 on all over the ring. Possibly a wave of insanity passed over the Saltwater River to put bookmakers and punters under a spell. But Ajax did not take long to break the spell. He took up his old role of pacemaker and just lobbed along in front without being seriously challenged.

3-: From the account of a boxing match, by a former boxer named Eddie Williams, published in The Sporting Globe (Melbourne, Victoria) of Saturday 1^st February 1941:

It wasn’t long before Fraser had Higgins hanging on the ropes like the washing. Tom Boyle shouted to Fraser to walk in and finish the job.
Fraser was carrying out instructions when Higgins threw a stupid looking swing from nowhere, and it landed on Fraser’s jaw. Down he went like stunned mullet, he didn’t come round for some minutes. It was the only time Fraser had been k.o.’d.

4-: From The Courier-Mail (Brisbane, Queensland) of Wednesday 16^th June 1943:

LEAVE DO’S AND DON’TS
From Allan Jones, Courier-Mail War Correspondent.

New Guinea, June 15.—A list of do’s and don’ts is handed to each soldier before leaving New Guinea for 21 days’ recreational leave in Australia.
Written in typical Digger language, the pamphlet says:—
“A few days ago you may have had in your mind that New Guinea was ‘on the nose’ where leave was concerned, but now you have changed your ideas on the subject and are flat-out, like a lizard drinking, thinking and making plans for a marvellous time.”
[…]
Among the “don’ts” are:—
“Don’t give the beer and spirits so much of a ‘nudge’ that you forget you are a man and a soldier.”
“Don’t get about your home town or any other place like a drip or a stunned mullet.”
“Any soldier with dirty boots, buttons undone, untidy dress, fag drooping at the corners of his mouth, chin strap behind his listeners, looks lousy, and is on the bugle.”

5-: From The talk of the Racecourse, by J. M. Rohan, published in The Sporting Globe (Melbourne, Victoria) of Wednesday 14^th June 1944:

A battler of olden days […] worked hard to get a dollar to put on the first winner. Luck came his way and his bank was £35 by the time it came to the certainty of the day—a 7/1 on chance. The battler had no hesitation in laying out his £35 to win £5. Seeing the good thing beaten left him like a stunned mullet. He was speechless. He couldn’t add to the roar of disapproval as the beaten favorite returned to scale.