Germany's Health Minister Ulla Schmidt faced fresh accusations of wasting taxpayers' money over revelations at the weekend she used a government plane to avoid a 90-kilometre car journey and save time.

Renewed criticism of Ms Schmidt's use of public money could embarrass her Social Democrats (SPD), who are lagging their conservative rivals by more than 10 points in polls as they seek to oust Chancellor Angela Merkel in a September vote.

The SPD share power in a "grand coalition" with their traditional rivals, Mrs Merkel's conservatives, who hope to win enough votes in the September 27 federal election to form a government with the business-friendly Free Democrats.

In July, Ms Schmidt's use of a chauffeur-driven Mercedes, which was stolen while she was on holiday in Spain but later found, sparked a row that pollsters say dented the SPD's ratings.

Newspaper Bild am Sonntag (BamS) reported Ms Schmidt in April had ordered a government jet to fly about 95 kilometres from Cologne to Maastricht while she attended an event in nearby Aachen so that she could use it to reach a connecting flight in Munich.

The paper said she could have driven from Aachen to Cologne instead, but Ms Schmidt's ministry said in a statement she would then not have had time to catch her Munich flight to Chicago.

FDP lawmaker Otto Fricke, head of the budgetary committee of the Bundestag lower house of parliament, told BamS questions about Ms Schmidt's use of the plane would need to be cleared up.

Karl Heinz Daeke, president of Germany's taxpayers' association, told the paper he could not fathom her actions.

"It's completely incomprehensible to me how someone can give so little thought to costs when planning a journey," he said.

The paper said flying the plane to Maastricht cost more than €2,000, and saved Ms Schmidt about 20 minutes. Questions also remain open about the use of her car.

After the controversy over the vehicle broke, Ms Schmidt said that she would repay the cost of the private use to the taxpayer - including having the car driven to and from Spain.

However, calls for a new investigation arose again when Ms Schmidt's ministry said she had also had her official car driven to Spain in previous years - and not reimbursed the taxpayer for any of the round trips of some 4,800 kilometres.