The former Australian army officer, who has spoken out often about human rights abuses in the country, presented a large Australian flag to Karen soldiers who have been fighting the Burmese army.
He spent a night at a base of the Karen National Defence Organisation, which has several thousand fighters, before re-entering Thailand near the border town of Mae Sot, 491 kilometres north of Bangkok.
Mr Simpkins did not pass through any official Thai or Myanmar border checkpoints when he travelled several kilometres into Burma, violating both its immigration laws and those of Thailand.
But Mr Simpkins defended the trip, saying it is important for outsiders to visit what the Karen call their sovereign state to ''see what is actually going on''.
''It is through international pressure that countries such as Burma with military-controlled governments will be forced to change,'' he said. ''We should be very careful about accepting everything they [Burma's authorities] say.''
The Thai army often turns a blind eye to visitors crossing the border into territory controlled by the Karen National Union.
The Karen mark ''Revolution Day'' on January 31, the day in 1949 from which they date their fight against Burmese rule.
In the intervening years the country's military has enlisted child soldiers and used rape as a weapon to terrorise the Karen, and many foreign MPs, NGO volunteers and journalists - including this reporter - have crossed the Thai border illegally to visit the camps of the Karen and other ethnic groups.
But since Burma's quasi-civilian government began releasing political prisoners, implementing reforms and opening to the world in 2011, Australia has upgraded its diplomatic relations, lifting travel and financial sanctions, increasing development aid to almost $90 million a year and posting a resident defence attache to the country.
The Burma government has negotiated temporary ceasefires with the Karen and other ethnic minority groups and wants them to sign a nationwide ceasefire on February 12, Burma's Union Day.
Mr Simpkins, who has represented the Western Australian seat of Cowan since 2007, argued in 2013 that Western nations, including Australia, moved too quickly to reward Burma's military.
''It is my view that the sanctions against Burma should be reinstated and that aid should be linked to human rights benchmarks,'' he told parliament at the time.
Burma's opposition leader, Aung San Suu Kyi, made similar remarks recently as the country's reform process appeared to stall amid concerns of inter-religious violence and persecution of the Rohingya Muslim minority in western Rakhine state.
Speaking in Thailand, Mr Simpkins said while not making the trip in his capacity as a federal MP, he intended to report his findings to Foreign Minister Julie Bishop. ''I am impressed by the Karen people and their determination . . . it's been a 66-year-struggle and a lot of people have lost their lives,'' he said.
Mr Simpkins said that if the government in Burma treated minority ethnic groups fairly in a true democracy ''then there would be no need for weapons . . . but until they are treated fairly and their nationalities are respected, the fight will have to go on.''
Mr Simpkins said there are several hundred Karen people living in his electorate who came from camps along the Thai border, adding they make a great contribution to Australia.
During a stay five-day stay in western Thailand, Mr Simpkins delivered support from Australian charities to 140,000 Karen living in nine refugee camps on the Thai side of the border, including from Wheelchairs for Kids, which is giving wheelchairs for landmine victims.
The Department of Foreign Affairs and Trade declined to comment on Mr Simpkins' crossing of the border.
Australian Federal Police are investigating the whereabouts of Northern Territory Labor Party chief Matthew Gardiner, also a former Australian army officer, who has reportedly joined Kurdish guerrillas in Syria fighting against the so-called Islamic State.
I had a Thai Karen (short neck) girlfriend who was one of the nicest people I have met in Thailand. Very polite, down to earth, although from a poor background not money grabbing and spoke very positively about Thailand which is her nationality. Karen people are much safer this side of the border she never said she was persecuted. She did not drink, smoke etc sweet lady.
Posted by Feisty Farang on February 3, 2015 18:06