using System;

using System.Globalization;

// Parse the input string to a DateTime, from a given format

DateTime dt = DateTime.ParseExact("20200424-04:09:42.145", "yyyyMMdd-HH:mm:ss.fff", CultureInfo.InvariantCulture);

​

// Convert the datetime from UTC to Sydney time

TimeZoneInfo dest = TimeZoneInfo.FindSystemTimeZoneById("AUS Eastern Standard Time");

DateTime transactTime = TimeZoneInfo.ConvertTimeFromUtc(dt, dest);

​

// Create and output the string you want to output, in a specific format

string output = transactTime.ToString("yyyy-MM-dd HH:mm:ss.fff", CultureInfo.InvariantCulture);

Console.WriteLine(output);

​