NSW Coroner Michael Barnes will on Wednesday hand down his findings into the December 2014 siege sparked by terrorist Man Haron Monis.
Police stormed the cafe early on December 16 after Monis shot dead Mr Johnson 17 hours after the siege started.
Police shot him dead and Katrina Dawson was fatally wounded by at least seven police bullet fragments.
Mr Johnson's mother Rosie Connellan says she initially believed the role of police in the events leading to her son's death was straightforward.
"We thought that Monis had killed Tori and, you know, it was relatively simple: the police had done everything possible and that was it," she tells ABC Four Corners program.
But she said the evidence at the inquest changed her opinion.
"As it unfolded it was just, it was horrific. Every day we thought: 'This can't get worse', and every day it got worse -- what was coming out in evidence," Ms Connellan said.
She was angry police waited until someone was killed before taking action.
"I'll never be able to understand how you can make a calculated decision that you wait for someone to die. It's just beyond me," she said.
Mr Johnson's partner, Thomas Zinn, said he had lost faith in the police.
"I have no trust because of the great level of incompetence that has been revealed," Mr Zinn said.
Ms Dawson's parents said they sat through the inquest to get answers, not retribution.
"(We wanted them) to admit the mistakes and then talk about what they would do to rectify those mistakes," Ms Dawson's father Sandy Dawson said.
"But we never saw anything like that."
The families hope the coroner's recommendations will bring about significant change.
"I want to see the coroner have the courage to really challenge an alternative view of what could have happened and to make sure the lessons are being learnt and that it honours the remarkable memories of Katrina and of Tori," Katrina's brother Angus Dawson said.
