The tragedy happened on the Praia dos Pescadores near Ericeira on the country’s west coast.
The head of the rescue service at Cascais port near Lisbon, Rui Pereira da Terra, told the Lusa news agency that it was suspected that couple had been taking a selfie on a wall above the beach.
He said: “Everything seems to indicate that the fall happened when they were probably trying to take a selfie.
“Since we found a mobile phone on the wall, everything suggests the victims might have been taking a selfie, when they allowed the phone to fall, leaned to grab it and fell.”
One Portuguese newspaper reported that fishermen discovered the couple’s bodies at about 6.30am.
The Jornal de Noticias reported: “At first the fishermen thought the corpses, from far away, might be people sleeping on the beach.
“Only when they got closer did they realise it was a dead couple and they were shocked with what they saw.”
The couple have not been named but some reports say the woman was aged 30 and the man 40.
A Foreign and Commonwealth Office spokesperson said: “We are in touch with local authorities following the death of a British woman in Portugal and are providing support to her family.”
The spokesperson did not confirm the nature of the incident or say how the woman died.
The Department of Foreign Affairs and Trade in Australia said it was providing consular assistance to the family of the Australian national who died.