My partner, who is Thai, has just become an Australian citizen. He is not supposed to hold both. We are going to Thailand. Should he travel on the Australian passport or should he use both passports? Would he leave and reenter on his Australian passport and enter and leave Thailand on his Thai passport? Would there be any problems at the Thai end?
If he has not formally renounced his Thai citizenship, then he is still Thai and should use his Thai passport to enter and exit Thailand.
He should use his Australian passport to exit and re-enter Australia.
I am not well versed in Australian citizenship law, but there is a reference on the government website that mentions a permanent resident of Australia could be come a dual citizen by becoming an Australian citizen, which to seems to me to mean your partner may be able retain his Thai citizenship - http://www.border.gov.au/Trav/Citi/Curr/Dual-citizenship