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Of course, comments on the Internet should not be a substitute for personalised advice from a qualified medical professional.
However, the authoritative source for regulation of drugs in India is the Central Drugs Standard Control Organization (CDSCO). The CDSCO has "control over the import of drugs, approval of new drugs and clinical trials" and other functions. This organization's website is ideal to look for more information; it is the most reputable source.
The CDSCO explicitly lists Venlafaxine Hydrochloride (Brand Name: Effexor) as a "Schedule H" drug available with a prescription in India (p. 407, No. 523). It is not a controlled substance.
The CDSCO explicitly does not list Lisdexamfetamine (Brand Name: Vyvanse) as approved or available in India. Because it is commonly abused, it is a controlled substance in Australia and many other countries.
Although border control officials have a great deal of immediate authority and discretion anywhere in the world, because the CDSCO regulates the import of drugs into India, it has approved Venlafaxine Hydrochloride, and the drug is not a controlled substance, it is a reasonably safe bet that it can be brought into India without trouble for personal use. Naturally, any prescriptions need to be in a small quantity consistent with the duration of travel and in an appropriately labeled prescription container along with a cover letter and a prescription from a qualified doctor. Bringing a highlighted printout of the cover and relevant portions from the CDSCO PDF would be a good idea, also.
Because the drug is approved by the CDSCO, it likely is quite possible to find a legitimate doctor in India who can prescribe Venlafaxine after a doctor's visit, as well. Based on personal experience with travel in India and other countries in South and Southeast Asia, even if you already have a letter from an Australian doctor, a pharmacist probably will refer you to a local doctor for a quick evaluation and a new prescription rather than fill a prescription from an unknown doctor overseas.
For Lisdexamfetamine (Vyvanse), because it does not appear to be approved by the CDSCO and it is a controlled substance, bringing it to India in any quantity would be highly risky. Bringing a large, multi-month supply of a drug that is not explicitly legal in a country and that is a controlled substance commonly abused very well could be trouble.
To reiterate, this is not medical advice, but you might wish to talk to your doctor about other ADHD medications that are confirmed legal and available in India before your travels.