Brendon McCullum has admitted Australia will still provide the toughest examination of all ahead of their two-Test series.

This is before the New Zealand wicketkeeper-batsman admitted he believes Australia are a "more vulnerable" side than ever during his international career.

The Black Caps go into the brief series, which begins at the 'Gabba in Brisbane on Thursday, as underdogs having not won a Test on their neighbours' soil in 23 years.

Their recent form hardly inspires either having struggled to overcome Bangladesh 1-0 in their Test series last month, before opening their Australian tour with an equally unconvincing six-wicket defeat to New South Wales.

Normally that would be enough reason to forecast doom for a side touring Australia, but the hosts are coming off their worst series defeat in 20 years after losing their four-Test tour of India 2-0.

The manner of that defeat has created a stir in Australia that the national team is not only in decline, following the retirement of several star names, but in danger of being surpassed by other nations.

Following the Kiwis' visit, the Australians face a home-and-away series against South Africa before an Ashes tour of England where some pundits predict they may also come unstuck.

McCullum's opinion on the Australians current plight is not so negative, however, with the veteran of 37 Tests claiming they remain the best team in the world.

"In comparison to the old Australian (side) they're definitely more vulnerable," McCullum said.

"It's the nature of losing three once-in-a-lifetime cricketers in (Adam) Gilchrist, (Shane) Warne and (Glenn) McGrath. Take that ability and class out of any side and it is going to take a little while for them to rebuild.

"They are more vulnerable but they are still far and away the best team in the world. They lost one series and they're being ridiculed in the paper for it, but in our minds they're still by far and away the best team."

McCullum freely admits the Kiwis face a tough task to take anything away from their series with the Australians, but he hopes they can use their underdog status to their advantage.

"We're at long odds but also with nothing to lose," McCullum said.

"You'll probably see guys play with a lot less fear than what we have done in the past. When you are such rank outsiders you can almost throw a little bit more caution to the wind."

The tourists were dealt a blow on the eve of the series when all-rounder Jacob Oram was forced to withdraw due to a back complaint.

McCullum has himself been troubled by a similar complaint, which flared again during the loss to New South Wales, although he is expected to play and retain his position higher up the order at number five.

Australia's preparations for the series have been marked by Andrew Symonds' return to their set-up.

Symonds has been out of the Australian team since August when he was sent home on the eve of the ODI series against Bangladesh for opting to go fishing instead of attend a team meeting.

The decision to expel Symonds had been made by the Australian players and as a result of that decision there had been speculation the Queenslander had fallen out with his team-mates including close friend, and Australia vice-captain, Michael Clarke.

Clarke dismissed such talk on the eve of the Brisbane Test, saying it was "really good" to have Symonds back in the team.

"It's always good to see 'Symmo' back around," Clarke said.

"It is really good having him back, and I think our relationship, as you guys would have seen today, we're going well."

Despite that Clarke stopped short of suggesting Symonds would automatically claim a spot in the Australian XI upon his return.

"Andrew's a fantastic player, he's been successful in all three forms of the game, so anytime that he was fit and ready to come back into the team, I think we're certainly going to have him," he said.

Selection for the match is set to be determined by storms that have hit the Brisbane area in recent days to leave the 'Gabba wicket with a healthy green tinge.

Both sides are therefore likely to opt for a mostly seam attack meaning Australia off-spinner Jason Krejza, who took 12 wickets on his debut in the fourth Test against India, may make way despite his impressive start to life in the national team.