Former Test batsman Kepler Wessels says Australia's disastrous one-day tour of South Africa is another blow to Steve Smith's captaincy.

The Proteas hold a 4-0 series lead with one match to play after a convincing six-wicket win over Smith's men at Port Elizabeth on Sunday.

Defeat in Wednesday's final match at Newlands in Cape Town would leave the tourists as the first Australian team to lose five matches in a bilateral ODI series.

Sunday's loss was Smith's eighth in his past 10 matches as captain in all formats since the start of the three-Test series in Sri Lanka in July.

During that period Smith was also sent home for a rest two games into the ODI campaign in Sri Lanka with David Warner then leading the team to victory in the three remaining one-dayers and the two Twenty20 matches on the island nation.

Wessels says the South Africa tour raises questions about Smith's leadership and that Australia was in danger of developing a defeatist attitude with their recent struggles.

"Getting into a losing habit is dangerous," the former Australia and South Africa player wrote in a column for broadcaster SuperSport.

"They no longer have the superstars in their line-up to change things around at will.

"They lost on his (Smith's) watch to Sri Lanka. After that loss he was given some time off to rest before this tour.

"In his absence David Warner led the one day team to victory in Sri Lanka. Clearly Smith will be feeling the heat now that he is back in charge and his team are 4-0 down in the series."

In 29 ODIs as captain Smith has won 16 and lost 12 with one no result.

That record is far from the worst for an Australian ODI captain, with Kim Hughes and Greg Chappell both losing over half their matches in charge in over 40 ODIs each.

But with Australia currently ranked No.1 in the world and as the reigning world champions, it's a record that is sure to raise some question marks.

Wicketkeeper Matthew Wade says the poor results in Africa haven't seemed to take too much of a toll on Smith however.

"He's a pretty chilled out captain. He's quite relaxed," Wade said.

"He obviously self-reflects away from the team but he keeps us all nice and calm. He's been good so far."