Eddie O'Sullivan has taken underlined his confidence in Ronan O'Gara as his side prepare to take on Australia in Perth on Saturday.

The Munster outhalf was blamed for the defensive lapses that ultimately cost Ireland the second Test against New Zealand, when All Blacks fly-half Luke McAlister brushed him aside in a five-metre charge for the line.

But O'Gara has been retained for the clash with the Wallabies, with O'Sullivan fulsome in his support.

"Ronan is not the biggest back in world rugby," O'Sullivan said. "He's an incredibly skilful player and runner of the game. What happens in modern rugby is that the opposition try to set up a mismatch with a guy who is 10 kilos heavier. It's a car wreck.

"And that's what happened on Saturday night. It's a bit like a dump truck hitting a bike. He keeps working on his defence but occasionally he gets caught."

While keeping faith with O'Gara, O'Sullivan has made one change at full-back, with Geordan Murphy dropped to the bench and Girvan Dempsey coming in for his 66th cap at the Subiaco Oval.

Announcing the team today, coach Eddie O'Sullivan stressed Murphy was not demoted for any particular failings in the recent defeats to New Zealand.

"I don't think he is on top of his game at the moment, and I think he would be the first to admit that," O'Sullivan said.

"I said at the start of the tour I would try to pick the best team each week."

Murphy remains on the bench to cover the back three, while Wasps fly-half Jeremy Staunton earns a place among the reserves, with veteran wing Denis Hickie omitted from the 22-man squad.

O'Sullivan is confident Ireland can match up to the Wallabies in Perth, and highlighted defence as the deciding factor if the weather is fine, as forecasted.

"I'm not trying to blow our own trumpet here, but we can play some pretty good rugby, we've got a good back line, and a good pack to give us the ball," O'Sullivan said.

"Both teams will be trying to run with the football, so I think it might boil down to which team has the best defence."

O'Sullivan denied Ireland would look to play a forward-dominated game in an effort to keep the ball away from the Australian backs.

"I wouldn't say that would be our whole strategy," he said. "We've got good backs as well, and we shouldn't not play them just to keep the ball away from Australia."