A moderate earthquake shook the top of the South Island early this morning.

The magnitude 4.5 quake struck just east of Seddon shortly before 4am.

It was about 21km deep.

A few people woken by the quake took to Twitter after they were jolted awake.

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"Insomnia problem #29 feel every small earthquake of the wee hours. #eqnz" wrote Samantha Jane.

"Go away. I just want to sleep. #eqnz" another Twitter user wrote.