Can anybody tell me if an Australian enters Thailand by way of Visa on Arrival , can he extend it without leaving the country and for how long is the extension.?Also after the extension can he fly out of the country and return again a few days later?

Can extend for 30 days in Thailand. Yes can can return, but depending on history, (how much time you have spent in Thailand, may face scrutiny). Have 20K baht with you upon return in case they ask for proof you have funds to live on.

An Australian does not qualify for a 15 day visa on arrival.

You can though get a 30 day visa exempt entry that can be extended for 30 days at an immigration office for 30 days for a fee of 1900 baht.

Without a visa you may be asked to show ticket out of the country within 30 days of arrival.

You could leave and re-enter the country and get a 2nd visa exempt after extending the 1st one for 30 days.

You could also consider obtaining a Tourist Visa (60 days) + 30 day extension = 90 days.

A single re-entry permit would cost 1,900 baht from Immigration to keep your Visa valid .

Thanks guys, I will weigh up all your advice and choose one that suits my mate's intentions.