Southee will decide if he's fit: NZ capt

Brendon McCullum is renowned for being the embodiment of the spirit of cricket but he's just as principled when it comes to teammates.

New Zealand spearhead Tim Southee suffered a back injury in the first Test at the Gabba, where he was unable to bowl in the second innings.

Southee is racing the clock to prove his fitness before Friday, when the second Test starts in Perth.

The 26-year-old will bowl in the nets on Thursday afternoon then see how his irritated disc feels on the morning of day one.

These sort of debates are quite contentious for most sides - the player, physio, doctor, selectors, captain and coach will all have strong views that don't always align.

That won't happen this time - if Southee says he is ok then he'll play.

"He's an honest bloke. He'll tell us," McCullum said.

"Tim is a strong leader within the group. We trust him completely.

"He's a team player ... the decision he makes will be the best for the team.

"If he is fit, I'm sure he'll be a force to be reckoned with in the game on a wicket that should provide him with a bit of bounce and swing."

If he isn't, Neil Wagner, Matt Henry and Mitchell McClenaghan are in the mix to replace Southee.

One of those quicks is expected to get a call-up regardless, with James Neesham ruled out of the series due to a back injury.

"There's certainly a consideration to play four quicks," McCullum said.

"Whatever happens, we're confident we're going to take in a team that's going to be able to get the job done.

"We've got some good options within our squad."

McCullum added that Southee would have to be close to full fitness to warrant selection.

"I don't think anyone's 100 per cent when they play this game," he said.

"He knows how to get himself up and get himself to the crease and still operate, even when he's not quite 100 per cent.

"He'll be cut a little bit more slack than someone we probably don't know as much about."

Meanwhile, McCullum backed swing specialist Trent Boult to improve on a poor showing in Brisbane.

"He wasn't at his best at the Gabba. There could be any number of reasons why," McCullum said.

"He's obviously coming back from injury. There's other circumstances too.

"The good thing about Trent is that emotionally he's a level bloke. He knows he's going to have the odd bad day but it won't affect his confidence."