Speaking to the BBC, Ms Forbes said people were watching the SNP "with astonishment" and party finance claims were "mind-blowing".
She said there was "time to sort it out" but "continuity won't cut it".
The SNP has ordered a review of how the party is managed following recent controversy over its finances.
Speaking last week, newly-elected party leader Humza Yousaf said he wanted a "fresh approach" to ensure party members, as well as the public, could be "really confident" in the governance and transparency of the party.
Since the shock resignation of Nicola Sturgeon as party leader and Scotland's first minister in February, the SNP has descended into turmoil.
The subsequent leadership race exposed deep divisions in the party, and midway through the contest Peter Murrell, Ms Sturgeon's husband, stepped down as chief executive after the party misled the media about membership numbers.
Mr Murrell was arrested earlier this month as part of a police investigation into the SNP's finances. On Tuesday, SNP treasurer Colin Beattie was also arrested in relation to the same investigation. Both men were released without charge pending further investigation.
Ms Forbes - who came second in the leadership contest behind Humza Yousaf - was speaking to the Radio 4 programme - Leading Scotland Where? which airs on Wednesday at 20:30 BST.
It is her first broadcast interview since the contest and was recorded after Mr Murrell's arrest but before Mr Beattie's.
Ms Forbes told the programme: "I think we need decisive action or we will be in trouble.
"People are watching with astonishment but they want to see the leadership dealing with it and resolving it."
She added: "Right now with questions over integrity, trust, transparency - I think voters are watching extremely carefully."
Looking ahead to the next general election - expected to take place in 2024 - she said people would vote on "the basis of how we have sorted out our internal problems - even more than that how we govern".
"There is still time to sort it out. But I said throughout the campaign, I'm afraid I'm going to say it now: Continuity won't cut it."
Asked about the way the party had been run by Ms Sturgeon and Mr Murrell, she said: "They were obviously a very good team in the sense of managing the SNP.
"But there's no question that since then there have been lots of questions about transparency... it doesn't matter how slick the optics are, you need good governance."
She added: "We are at a pretty critical moment - and it will be the response and the reaction that determines how big a problem this is for the SNP."
Ms Forbes dismissed calls made by some in the party for a re-run of the leadership election.
But she suggested she could have won if the campaign had been longer.
She told the BBC: "One argument I think does have merit is that the contest was so short.
"I came from a standing start, I hadn't been in front line politics for about seven months, came right into the full glare of media scrutiny and the requirement to build a team and also build a policy platform pretty quick.
"There are some who have argued who I would probably agree with that if the contest had been longer each candidate would have had more time to connect with the electorate."
Asked if she thinks she could have won, she replies: "Yeah, there's only 2,000 votes in it. But then again I also have confidence SNP members know who they are voting for."
Despite calls for unity, Ms Forbes left the cabinet in Humza Yousaf's reshuffle.
At the time, the deputy first minister Shona Robison suggested this was for a better work life balance.
But Ms Forbes said: "The primary reason that I didn't take the job was because I couldn't do positions that I'd taken during the campaign.
"Having made much of integrity - I think it was important to be able to hold to those positions.
"I know how important it is within cabinet to work together and support the decisions made."
Ms Forbes did not rule out running for the leadership again in the future but said it was "highly unlikely".
She also said she would be a loyal backbencher to Mr Yousaf.