I have a valid B1/B2 visa valid to US till March 2019 on Indian passport, I changed my citizenship to Australian. Is my B1/B2 visa on Indian passport valid?
You probably have noticed that India does not recognize dual-citizenship so your Indian passport is no longer valid (as far as India is concerned) and should be given back to the nearest Indian mission. You are facing - penalties otherwise.
Also since you have an Overseas Citizenship of India card, India is aware that you acquired another citizenship and in an extreme case might have listed your passport as invalid in international databases.
Given all that, when traveling on an Indian passport, you are making the false claim that you are an Indian citizen.
To do everything in a perfectly legal way, you should get a new U.S. visa with your Australian citizenship/passport. Note that you are eligible for the Visa Waiver program (under conditions) but you say your intended stay is longer than 90 days, so you should get the visa. You can of course try asking the responsable American mission if they can just transfer your visa, however I somewhat doubt they might be able to do so.
In practice, possibly or likely your Indian passport will be fine until it expires and you can try to use it to enter the U.S. In that case see this post for a discussion on how to travel as a dual citizen:
However I am not sure if your new citizenship constitutes a material change of circumstances from the point of view of the U.S. and if they care about it. We had a similar question last year
and answers do not indicate that would be an issue, but if you are completely risk-averse on visa issues as I am, as mentioned above, get a new visa. On the other hand, the CBP says:
Individuals who possess a valid visa will still be able to travel to the United States on that visa for the purpose for which it was issued. Individuals traveling on valid visas are not required to apply for an ESTA authorization.
If you are no longer a national of India then your old visa is no longer valid.
It is possible to use a valid visa in an expired passport, but the visa indicates that you are an Indian national, so your loss of Indian nationality causes the visa to be invalid.
When you travel to the US, you should do so as an Australian, either by applying for a new B1/B2 visa or by using the Visa Waiver Program.