Peter Siddle's deadline is fast approaching as the paceman races the clock to prove his fitness for the second trans-Tasman Test.

Siddle will almost certainly need to prove he's ready to go by Thursday to warrant selection in the team which can regain the No.1 Test ranking if they win or draw the clash in Christchurch starting on Saturday.

Cricket Australia is upbeat the back spasms experienced by the 31-year-old during the first Test in Wellington aren't anything too serious.

That was the case earlier this summer, when Siddle was restricted by a similar issue in the day-night Adelaide Test.


The Victorian was able to back up and play the next Test but he enjoyed a 10-day break between games.

This time, there is a buffer of only four days, one of them a bonus after the tourists completed an innings-and-52-run victory on day four in Wellington.

The squad travel to Christchurch on Wednesday, leaving Thursday as the main session when Siddle will need to prove his fitness.

"We'll wait and see how he pulls up in the next couple of days," coach Darren Lehmann said.

"He'd have to bowl pretty well in the days leading up.

"He'll have to recover pretty quickly but we'll just wait and see.

"If he's not a hundred per cent, he won't play - that's what we do."

James Pattinson can attest to that.

Pattinson was keen to play the first Test and impressed in the Basin Reserve nets last week but selectors and team medical staff harboured doubts about his match fitness.

The paceman trained throughout the first Test, looming as the man most likely to take Siddle's place in the side.

"He is pretty close (to being ready for Test cricket) - as long as he gets through the main training session," Lehmann said.

Jackson Bird, despite failing to fire on day one in Wellington, is not expected to come under selection pressure, unless Siddle is passed fit.

"He got better throughout the game," Lehmann said.

"He was a bit short and a bit wide and a bit nervous in the first innings, as you would be, having not played since Durham in 2013.

"A little bit of nerves but he got better as the game went on, which was good for him."

Settling on a pace attack shapes as crucial to Australia's hopes of topping the International Cricket Council's Test rankings for the first time since 2014.

It is also one of few concerns for Lehmann after a polished performance in Wellington.

"Our patience with the ball .... our catching ... the way we batted," Lehmann said, when asked what pleased his most about the win.

"The challenge is to back that up next week in Christchurch."