Police are searching for a man who tried to have sex with a dog in front of two dog owners in Sydney's west on Sunday.

Two women, aged 20 and 23, were walking their two dogs through Greystanes about 1pm on Sunday when they were approached by the man, who was riding a pushbike.

Police released an image of the bike. Credit:NSW Police Media

He stopped to talk to the women on Canal Walkway and assaulted one of the dogs in an indecent manner, a police spokesman said.

He then exposed his penis in front of the women and "attempted to commit bestiality on the other dog", the spokesman said.