Georgina and Caroline Evers-Swindell, three-time world champions and the 2004 Olympic gold medallists in the double sculls, suffered the ignominy of failing to reach the A final in the World Cup regatta at Poznan.
They came last in their heat and last in their repechage before winning the B final last night (NZT).
New Zealand won gold medals through world champion single sculler Mahe Drysdale, double scullers Rob Waddell and Nathan Cohen and the men's pair of George Bridgewater and Nathan Twaddle.
As well, the Olympic preparations of other crews looked on course as the women's pair of Nicola Coles and Juliette Haigh finished second in their A final, as did lightweight sculler Duncan Grant, while the emerging Emma Twigg was third in the women's single sculls.
Offsetting those positives were the performances of the Evers-Swindells and the men's four of Hamish Bond, Eric Murray, James Dallinger and Carl Meyer, the 2007 world champions, who were fifth in their final.
Tong, a coach and a bronze medallist in the men's coxed four at the 1984 Olympics, said the twins had time to put things right before the Games start in Beijing on August 8.
Having missed the Lucerne World Cup regatta this month after Georgina exhibited flu-like symptoms, Poznan represented the sisters' last chance to compete among international crews ahead of the Games.
"Knowing the twins and knowing Dick (coach Richard Tonks), they will be working harder to make things come right," Tong told Radio Sport.
"The concerning thing is that the way they used to race was that they were very, very strong over the first 500m.
"It makes me think something is wrong; whether it's a technical problem or a health issue, they just did not fire at all in the first 500. They were missing their usual spark."
Tong said he remained confident in the Evers-Swindells because they had fought back from adversity before.
"If they were a race horse you could say they were a couple of runs short of their top form.
"We know they can handle this sort of situation. They will be putting more pressure on themselves than anyone in the New Zealand public can put on.
"They are the ones who want to perform.
"I think they will come right, they will get into the (Games) final and I'm confident they will be competitive in that final.
"We know they are not at their peak but we also know they can go fast enough to beat everyone they race against."
Tong pointed out that the men's four, while fifth, were just 4.5 seconds behind the winning Dutch crew in the final, which they qualified for via the repechage.
"You can turn that around in a couple of days.
"You are only looking at one or two percent difference in boat speed, that equates to five or six seconds.
"They lost four seconds in the first 500m, and if they sort that part of the race out and they'd be right there."
New Zealand has qualified eight boats for Beijing.
The 16 rowers will spend time before the Games training at Lake Karapiro, near Cambridge, or in Sydney at the venue used for the 2000 Olympics.
