Olmert, who once led the centrist Kadima party, told Israel's Channel 2 television on Friday that 11 billion shekels (about $3 billion) were wasted on "illusionary security escapades that have not been implemented and will not be implemented".
Olmert, prime minister from 2006 to 2009, did not mention Iran by name but Israeli media said his meaning was clear in the attack on Netanyahu in the run-up to a January 22 parliamentary election.
Israel and the West suspect Iran's nuclear program is aimed at developing atomic weapons. Tehran says it is enriching uranium for peaceful purposes only.
"Last year they (Israel's leadership) frightened the whole world and in the end nothing was done," Olmert added, appearing to allude to warnings by Israel that it might strike Iran if Western sanctions failed to curb its nuclear activities.
Netanyahu, who leads the right-wing Likud party and is widely forecast to win the election, has set out a mid-2013 "red line" for tackling Iran's uranium enrichment project.
Asked by Army Radio about Olmert's comments, Netanyahu said: "This is a strange and irresponsible statement. I will not specify the sums of our defense expenditure.
"I will say that we have developed offensive and defensive capabilities for close and distant theatres and I think that this is a very important investment for the state of Israel."
He reiterated that Israel "must do everything in its power to prevent Iran from getting nuclear weapons", a goal he said would be his "number one task" after his re-election.
Olmert, who is not running in the ballot, resigned as prime minister in a corruption scandal and was convicted in July on a relatively minor charge of breach of public trust, receiving a one-year suspended sentence.
He is still a defendant in a bribery trial related to a Jerusalem real estate project.
(Writing by Ori Lewis; Editing by Alison Williams)