Australia police
The teenager is said to have started using the knife when his 36-year-old stepfather was "throttling" his seven-year-old sister - Representational image REUTERS/David Gray

An 18-year-old man from Perth, in Western Australia, has pleaded guilty to killing his stepfather in a knife attack which started in order to save his seven-year-old sister, the WA Supreme Court has been told.

The teenager who cannot be identified because he was only 17 when the incident took place is said to have stabbed his stepfather 25 times at their northern suburbs home in September last year.

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The teenager was originally charged with murder and was due to stand trial next year. The prosecutors have now accepted his guilty plea and charged him with a lesser charge of manslaughter.

The teenager started using the knife when his 36-year-old stepfather was "throttling" his seven-year-old sister, the court heard, according to ABC Online.

The teenager reportedly continued to stab the man when he turned his attention towards him.

Prosecutor Bernard Standish said the attack "started off being defence of another and then turned to self-defence", but because of the continued stabbings, the teenager's response "became excessive".

According to prosecutor Standish, there was "history" between the people involved in the incident. He said there had been a previous confrontation between the teenager and the victim.

The teenager has been released on bail until he faces a sentencing hearing in February. Justice Stephen Hall has ordered that before any reports are prepared for the hearing, there is a need to examine the background of the relationship between the teenager and his stepfather.