For people living in the vicinity of Sai tai mai (new Southern Bus Teminal) Bus no 66 (orange with AC & beige/red without AC) brings you to Government Complex Chaeng Wattana ... the bus stop is right in front of the building, where the immigration is located.
TLDR : Answer Summary
To reach the Government Complex Chaeng Wattana from Sai Tai Mai (New Southern Bus Terminal), you can take Bus No. 66, which is available in two types: an orange bus with air conditioning and a beige/red bus without air conditioning. The bus stop is conveniently located right in front of the immigration office in the complex.