I was thinking of flying into bkk from Australia with no visa (get it on arrival) and then stay a week and then fly to india and then return to BKK. When I return to BKK will I have issues at immigration if I don’t have an onward ticket?

Possibly, but not likely. Your biggest problem will be the airline, who are likely to refuse boarding without an onward ticket.

Don't forget you need a visa for India

Where in India are you going, there are a number of consulates so getting a tourist visa there would be an option, definately zero issues with onward flights then.

Alternatively get a tourist visa before setting out from Oz. You can ask immigration at BKK not to use your TV for your initial entry (get a 30 day stamp) and save the visa for your return.

As already mentioned your biggest problem willbe when checking in. In the past when i have done this, i have always had problems and managers have been called down etc. Once this even hapened when i had Non imm B for one year as i was one way from UK too BKK.On check in i was asked why i did not have an outward bound ticket.

I was refused check-in by Qantas on a flight from MEL-BKK because I didn't have an outbound ticket from BKK. What really pissed me was the fact I had a valid retirement visa for Thailand but they argued it only had two days remaining and it might not get renewed. No amount of talk could change their mind so I was forced to buy a one way refundable ticket BKK-Singapore so I could check-in.