i dont think you are covered under employment act
And your legilitive complain period is one month of termination of your contract.
so that means. now you have to play under their rule. forget about the employment act. but just the "contract" you signed on with them. and all evidences during ur resignation period.
i assume you have a written resignation letter, right? it has date and all those things. if they did not sign as receive notice, which means you can simply claim they fired you instead of you resigned.
Lastly try to think from their side... what do they have to prove you are guilty or mis-conducted.
try to check you resignation process against your contract with the firm. if everythihng make sense but lack of evidences. dont worry. that means they dont have neither. so just be cool. ignore what they are doing now. they are just bluffing.
http://www.mom.gov.sg/publish/momportal/en/communities/workplace_standards/employment_standards/the_employment_act/Who_the_Employment_Act_Covers.html
"senior software engineer" : Managers and executives are employees with executive or supervisory functions. These functions include the authority to influence or make decision on issues such as recruitment, discipline, termination of employment, assessment of performance and reward, or involvement in the formulation of strategies and policies of the enterprise, or the management and running of the business.
They also include professionals with tertiary education and specialised knowledge/skills and whose employment terms are comparable to those of managers and executives. Professionals such as lawyers, accountants, dentists and doctors whose nature and terms of employment are comparable to executives would generally be deemed as such, and hence they would not be covered under the Act