The Australian coaching staff has expressed their frustration at the apparent joint-decision by Steve Smith and umpires to play an extra half-hour in Sunday's never-ending final session that now leaves day five at the Gabba hanging on a knife's edge.

Assistant coach David Saker made it clear that he and head coach Darren Lehmann were riding every minute of the agonising day four struggle as Pakistan edged ever closer to a miracle, even admitting to privately cursing skipper Smith up in the box for his two uncharacteristic dropped catches.

In a brutally honest press conference after what was an extraordinary day's play, Saker also took aim at the pink ball's tendency to go soft as a contributing factor in Pakistan's unlikely charge towards a world record run chase, with the visitors needing a further 108 runs with two wickets in hand to reach their mammoth target of 490.

The laws of the game state that the umpires can only choose to extend play if requested by one of the captains, and it would seem Smith decided to keep hammering away at Pakistan who were seven down, even though the final session was pushing past the three hour mark.

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If not for Jackson Bird's dismissal of Wahab Riaz in the final over the decision would have been a complete disaster, but even so, plenty of damage was done with 51 runs coming in the extra half-hour - and 179 overall in the last session overall.

Smith's request didn't mean the umpires had to agree - and Pakistan felt it was harsh on them to be forced to bat on for a further eight overs given they'd already come so far under lights - but the fact is Australia had to ask the question in the first place.

Saker and Lehmann were far from pleased.

"I actually don't know what happened. It was a bit frustrating from where we were sitting," said Saker.

"We were quite happy to come off as a support staff because I thought we'd done quite a bit of work and we'd probably rather our fast bowlers put their feet up and we weren't sure what was going on.

"I know the umpires and Steve were talking out there but I haven't had a chance to talk to Steve about what happened.

"Obviously they've just given the half-hour extra which is allowable and I suppose with the new batter in that probably wasn't the right decision but a lot of people wanted the game to finish today and the umpires were probably (two) of those."

Smith did redeem himself with a sharp catch off Bird in the final over, but before that he had dropped two catches he normally would have swallowed.

The second one was particularly costly, as it gave super No.6 Asad Shafiq a life which enabled him to carry on and make a brilliant unbeaten hundred.

Disappointed at the ball and the line bowled by his pacemen at times, Saker said emotions were getting increasingly heated in the rooms.

"He's obviously a very good slips fielder and he dropped two catches he'd usually catch," said Saker.

"It was a bit frustrating in the dressing room as well. I think, he wouldn't want to be in our rooms with Darren and I going off our ... but that's the way it's and that's cricket and he's usually a very safe pair of hands.

"In the end he ended up taking a pretty good one at the end which is a pretty important wicket in the context of the game."

As the boundaries started flowing from Shafiq and lower order comrades Mohammad Amir (48) and Riaz (30) at the death, Saker admits Australia's minds turned to the worst-case scenario of losing a match by a world record.

However, he insists his team still hold the upper-hand.

"You think you're just one wicket away from breaking that and we're still in a very strong position as I said," he said.

"It's been a pretty heroic run chase from them. The highest run chase at the Gabba in a fourth innings. The wicket is a traditional Gabba wicket which with the red ball I think would offer a bit more pace and carry. (But) with the pink ball the life goes out if it a bit earlier than the red ball.

"There's no excuses for the way we went about it but that's probably one of the reasons they've scored the runs they did. They batted particularly well."

Asked about the verbal stoush between Mitchell Starc and Amir, experienced bowling specialist Saker offered a unique take.

"It might have had something to do with his knee injury on the first day I'm not sure," said Saker of an earlier incident where Amir was stretchered off the field with an apparently serious knee injury only to return a short time later.

"The recovery powers were quite unique so he might have been asking how he recovered so quickly."