I have applied for Subclass 600 (tourist) Visa on Jan 30th, 2019. The processing time was showing 15-27 days when I submitted the application. Today when I logged into my immi account now its showing 18-29 days. I have got no information about this processing time change in my immi portal nor over the internal. Is this very usual or something happening only with me?
The processing time displayed is calculated from the actual processing time of applications submitted in the preceding few weeks. It will vary, and in a few days it may be longer or shorter than you see today.
You can find the full details on the Australian Immigration web site
You should hear something in four to six weeks. If you haven't, contact them again.