The presence of more than 1500 US marines in Darwin should be welcomed and is a "really positive engagement", Defence Minister Marise Payne says.

Senator Payne made the comments after viewing a computer simulation exercise involving Australian and US forces in which bombs explode in Darwin's Cullen Bay and the troops respond during a visit to Robertson Barracks in the Top End.

The simulator can be used for mission rehearsal exercises for any scenario before going to an "operational or live firing environment", Australian Army's Major Max Williams told the minister.

The number of US marines in Darwin has been gradually increasing to 2500 since first being stationed in 2012.

There are 15 different joint training exercises planned over the next six months.

The meeting comes at a time of increased tension between China and both the US and Australia. The US withdrew an invitation to China to attend the Rim of the Pacific naval exercises this year and US Secretary of Defence James Mattis warned Beijing during a speech in Singapore last weekend of consequences over its aggression in the South China Sea.