A gunman armed with an assault rifle killed 20 people and wounded 26 at a crowded Walmart in El Paso, Texas before being arrested on Saturday.
"So terribly saddened by the senseless violence in El Paso that has taken so many innocent lives," Mr Morrison tweeted on Sunday.
"Our thoughts and prayers are with the families and communities impacted as well as the first responders. A terrible and evil attack."
It was the 249th gun massacre this year in the United States.
Former deputy prime minister Tim Fischer said Americans must stand up to the gun lobby after the latest atrocity.
"Still nobody directly calls out the real problem and the National Rifle Association policies and priorities and its controversial CEO Wayne LaPierre," Mr Fischer told AAP on Sunday.
"Until the state and federal congresses stand up to the NRA no real progress will be made.
"In the meantime Australia's DFAT should upgrade the smart traveller site to reflect the dangerous situation in the USA re guns."
Mr Fischer has called on journalists to bail up US Secretary of State Mike Pompeo who is in Sydney on Sunday and ask directly why tourists should risk their lives in travelling to the USA.
He said Mr Pompeo should also be asked why the US doesn't at the very least adopt background checks for gun purchases and limitations on assault weapons.
Mr Fischer was instrumental in helping the Howard government introduce sweeping gun reforms and an amnesty in Australia after the Port Arthur massacre in 1996 when 35 people were killed by a single gunman with a semiautomatic weapon.
