Nanomaterial based artificial enzymes or nanozymes exhibits superior properties such as stability, cost effectiveness and ease of preparation in comparison to conventional enzymes. However, the lower catalytic activity of nanozymes limits their sensitivity and thereby practical applications in the bioanalytical field. To overcome this drawback, herein we propose a very simple but highly sensitive, specific and low-cost dual enhanced colorimetric immunoassay for the detection of avian influenza virus A (H5N1) through facile in situ synthesis of gold nanoparticles and their peroxidase-like enzymatic activity. 3,3-5,5 Tetramethylbenzidine (TMBZ) was used as a reducing agent to produce gold nanoparticles (Au NPs) from a viral target-specific antibody-gold ion complex. The developed blue color from the sensing design was further amplified through catalytic activity of Au NPs in the presence of TMBZ hydrogen peroxide (H202) complex. The developed dual enhanced colorimetric immunosensor enables the detection of avian influenza virus A (H5N1) with a limit of detection (LOD) of 1.11 pg/mL. Our results further confirms that the developed assay has superior sensitivity than the conventional ELISA method, plasmonic-based bioassay and commercial flu diagnostic kits.