A tough couple of hours loom for New Zealand rowers Rebecca Scown and Genevieve Behrent at the Rio Olympics.

The pair won their heat to move directly to their semifinals, winning by two seconds from South Africa.

However they are also part of the women's eight, who must take a harder route if they are to stay alive in the regatta.

The eight were second in their heat, clocking 6min 12.05s and that sent them into a repechage.

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The problem is their pair semifinal is to be raced at 11.30pm NZT on Wednesday, 80 minutes before their eights repechage.

It will be a substantial test of their stamina to be at the peak of their powers for the eights race.

At their last pre-Olympic regatta in Poland, coach Dave Thompson swapped the experienced Scown out of the stroke seat and she changed places with Emma Dyke. It was thought it was at least in part to take a little strain off London bronze medallist Scown.

The first New Zealand crew to drop out of contention, of the 11 in Rio, are the men's quad, who finished last in their repechage, seven and a half seconds behind winners Germany.

The quad of Nathan Flannery, Jade Uru, George Bridgewater and John Storey, always faced an uphill job, having been a late entry into the field. They had missed selection at the final qualifying regatta in Lucerne in May but got in when the Russian quad were booted out after an athlete failed a doping test.