The Black Caps' poor form in Australia has left captain Kane Williamson scratching his head in frustration.

Black Caps skipper Kane Williamson and batsman Henry Nicholls have both been sent back to the hotel from training two days out from the third cricket test against Australia in Sydney.

Williamson and Nicholls rocked up to the SCG for training but both were sent back to the team's hotel suffering from "flu-like symptoms", according to team officials, with medical staff keen to keep them away from other players.

There is no suggestion, at this stage, that the pair is in danger of missing the third test at the Sydney Cricket Ground.

ANDREW BROWNBILL/AP Henry Nicholls has joined skipper Kane Williamson in being sent back to the team hotel today.

"They're fine. They're not really feeling good today with some flu symptoms and I'm sure they will be back training tomorrow. They're pretty tough, so just take a rest today. It's been a big last two tests so it's a good opportunity to have a day off and come back tomorrow," bowling coach Shane Jurgensen said after training.

It is an optional training session today and one player who decided not to train was Neil Wagner, who has earned a day off to recover after mammoth bowling stints in the first two tests in Perth and Melbourne where he has been New Zealand's standout player.

New Zealand trail the three-test series 2-0 after being hammered in the first two tests.

The third test starts on Friday.