The 17-year-old Launceston Christian School year 12 student recorded a comfortable victory on the national stage, recording a personal best time of 54.95 seconds.

Her time broke the qualifying mark for next year's world junior championships.

``I'm ecstatic because I wasn't expecting to break 55 at all,'' de la Motte said.

``It came completely out of the blue but I suppose it shows that I'm in shape which is good because it was a PB by almost a second.''

De la Motte finished second in the 400m at the national junior championships in March and yesterday's victory was her first national gold medal.

TIS athletics co-ordinator and coach Peter Fortune said it was a terrific run with his charge winning by about 1.5 seconds.

``She has been going pretty well but probably needed the right sort of race to go a little bit better than she has been and she got the good race today and it worked out well,'' Fortune said.

``It is the world junior qualifying time for 2012 for the games in Barcelona in July.

``She still has the national selection trials in March, but she is the only girl to qualify in the 400m so far.

``If she wins the selection trial she would be pretty much automatically selected.''

De la Motte ran 55.86 to win the 400m at a national under-20 relay development camp in Hobart last weekend.