What are income tax plates in Australia

What are the tax implications of selling an investment property in India?

Do I have to pay tax on my 15 lakh winnings?

Yes, you have to pay capital gains tax. This transaction is not exempt as the profits are incurred in India and are therefore also taxable for NRI.
If the property is held for less than 3 years this is a short term gain and will be taxed according to your income tax plate.
With a holding period of more than 3 years, this would be long-term profits of 10% without indexing or 20% with indexing.

Can I transfer money from my NGO to an NRE account?

No. Credits to NRE accounts are only valid for certain eligible transactions. Funds cannot generally be transferred from NGOs to NREs.

If you bought the house by debiting your NRE / FCNR account or took out a home loan and paid it back from the NRE / FCNR account [mainly from overseas transfers], you can repay the same amount. So if you have debited the NRE account for 35 lacs, you can get 35 lacs back in FCY.

The winnings can also be returned under a total limit of USD 1 million. So you need to bring the money back to the UK and then send it back to your NRE account.

See the RBI policy