My family is travelling from Kabul to Sydney via Delhi airport with Air India all the way. We all have Afghani passports with Australian visas in them. We have a 19 hour layover in the Delhi airport. Do we need visas if we stay inside the airport and do not go outside?
Given that you're flying with Air India all the way, chances are it's on a single ticket.
In this case, the rules for transit as stated in Timatic apply, namely:
TWOV (Transit Without Visa): Holders of confirmed onward tickets for a max. transit time of 24 hours
So no, you do not need an Indian visa.