Senator Johnston says subs will take time

Unlike former prime minister Tony Abbott, onetime defence minister David Johnston says he's not flabbergasted at the prospect of the first new sub arriving in the early 2030s as he knows just what's involved.

Senator Johnston, appointed defence minister after the coalition won the 2013 election, said building new submarines was Australia's equivalent of landing a man on the moon.

"The design phase will take seven years minimum. Then the construction phase will take another seven years and the commissioning phase will take several more years, probably two to three," he told Sky News.

Earlier this week, Mr Abbott said he expected the new subs to be in service in the mid-2020s and was flabbergasted at this delay.

Senator Johnston said he wasn't flabbergasted as he understood what was involved in the project.

"Any delay is going to be because useful work is being done on what is going to be for Australia putting a man on the moon," he said.