With 100 days to go before co-hosting the World Cup with New Zealand, the Matildas returned to winning ways after their friendly defeat by Scotland on Friday thwarted a seventh straight win.
"I think most of all they (Australia) were very efficient. We gave them chances," Wiegman told ITV.
"In possession, we were too slow. You need dynamic movement to stretch them, we didn't do that well enough."
Kerr opened the scoring in the 32nd minute capitalising on a poor clearance by defender Leah Williamson to coolly slot home, despite appearing to be offside as there was no VAR in place.
"England's such a big footballing nation, hopefully, there are some happy Aussies, but we're not getting ahead of ourselves," Kerr said.
Australia's counter-attacking play paid off and Grant doubled the lead in the 67th minute with a header that bounced off Williamson, who had a night to forget, and into the net.
"We looked like we lacked ideas on the ball, and we got punished. It was my mistake for the first goal, and they were ruthless on the counter," Williamson said.
"I'm not one for dwelling on personal things, that first goal has made it worse for me but the whole team feels really rubbish about losing. It hurts.
"Absolutely not a setback (for the World Cup). For us to lose tonight gives us a bit of fire."
The Lionesses pressed but could not find a way past a solid Australian defence, as they lost for the first time under Wiegman's tenure.
"This is the first time we lose, but we always learn," Wiegman added.
"We have to improve to be at our best at the World Cup. Every game is for learning, and this is a big one."
Australia play France in a friendly on 14 July, before their World Cup opener against Ireland six days later.
England kick off their World Cup campaign against Group D opponents Haiti on 22 July.