Jakarta officials handed the Australian pair their execution notices on Anzac Day despite repeated pleas for clemency by the Australian government, international human rights lawyers and prisoners' families.
Mr Tuck said he first met Chan and Sukumaran at Kerebokan prison in January when he visited his client, Whanganui man Antony de Malmanche, 52, who was arrested in December after allegedly bringing 1.7kg of methamphetamine into Bali from Hong Kong,
De Malmanche is on trial and is due back in court today if well enough to do so. He had suffered four angina attacks since being imprisoned, Mr Tuck said.
"I have spoken to Chan and Myuran several times and our legal team also helped provide their lawyer with some legal points to assist with the appeal process. It's absolutely gut-wrenching news. Their lawyer will be devastated."
Mr Tuck said Chan had assisted de Malmanche while he was in Kerebokan.
"Andrew and Myuran were impressive young men who unfortunately dabbled in drugs when they were much younger and had already paid the price by spending a very long time in prison. To put them to death just seems so frightening, brutal and unnecessary. It's feels so unreal and does your head in just thinking about it."
Mr Tuck said his clients and other prisoners in Kerebokan would be feeling "very vulnerable".
"It is very unsettling time for everyone at the prison and the fear and pressure Tony and Lindsay are feeling right now is immense," he said.
Mr Tuck is also a member of the international legal team trying to save the life of British grandmother Lindsay Sandford, 58, by mounting a last-ditch appeal after she was found guilty of smuggling $3.3 million worth of drugs into Bali.
Ms Sandford, who is also in Kerebokan prison and has assisted de Malmanche and other prisoners, is also facing the firing squad, he said.
Mr Tuck is also acting for Peter Gardner, the young New Zealander and Australian citizen, who has been detained in China accused of trying to smuggle 40kg of methamphetamine out of China
Gardner's trial was set to be begin on May 7 and was expected to take two days.
Mr Tuck said he was due to fly to China later this week.
Ms Sandford and de Malmanche have condemned the executions of the Bali 9 ringleaders, saying they did not deserve to die.
In a written statement sent to the Bay of Plenty of Plenty Times, Ms Sandford said: "I am deeply saddened to learn that Myuran Sukumaran and my dear friend Andrew Chan have been executed.
"The Andrew and Myuran I knew were men who did good and touched the lives of a great many people, including myself."
From Parliament yesterday, Bay of Plenty MP Todd Muller strongly condemned Indonesia's application of death penalties.
"I do not think it appropriate in today's world as a form of punishment regardless of the crime," he said.
additional reporting NZME.
