World Cup debutant Walczak had to settle for fourth in the women's lightweight single scull, in contention for the first 1300m before slipping off the pace as Canadian Jaclyn Halko took the win.
Fellow debutant Kirkwood was also in conention in the men's lightweight single scull final for the same amount of metres but finished one place lower in fifth with New Zealand's Duncan Grant victorious.
Earlier in the day all four of Britain's women's crews were successful in their repechages with Jess Eddie and Katie Greves second behind hosts Australia in the women's pair.
They join heat winners Helen Glover, the Olympic champion, and Polly Swann in the final while Imogen Walsh and Kathryn Twyman won their lightweight double scull repechage race.
Victoria Thornley and Beth Rodford were also winners in their double scull repechage and the British quadruple scull progressed through after finishing third in their outing.
Britain will have a total of 14 crews race in 12 further finals with three of the four Brits that won Olympic gold in the four at London 2012, Peter Reed, Andrew Triggs Hodge and Alex Gregory, racing in the men’s eight in Sydney.